Early reports of Thunderbolt 5 performance have been mixed. This article starts from Thunderbolt 3 and dives deeper into the TB5 specification to explain why it might not deliver the performance you expect.

Origins

Thunderbolt 3 claims to deliver up to 40 Gb/s transfer speeds, but can’t actually deliver that to a peripheral device. That’s because TB3 includes both 4 lanes of PCIe data at 32.4 Gb/s and 4 lanes of DisplayPort 1.4 at up to 32.4 Gb/s, coming to a total of nearly 65 Gb/s. But that’s constrained within its maximum of 40 Gb/s.

The end result is that the best you can expect from a Thunderbolt 3 SSD is a read/write speed of around 3 GB/s. Although that would be sufficient for many, Macs don’t come with arrays of half a dozen Thunderbolt 3 ports. If you need to connect external displays, the only practical solution might be to feed them through a Thunderbolt 4 dock or hub. As that’s fed by a single port on the Mac, its total capacity is still limited to 40 Gb/s, and that connection becomes the bottleneck.

Thunderbolt 5 isn’t a direct descendant of Thunderbolt 3 or 4, but is aligned with the second version of USB4. This might appear strange, but USB4 in its original version includes support for Thunderbolt 3. What most obviously changes with USB4 2.0 is its maximum transfer rate has doubled to 80 Gb/s. But even that’s not straightforward.

Architecture

The basic architecture of a Thunderbolt 5 or USB4 2.0 connection is shown above. It consists of two lanes, each of which has two transmitter-receiver pairs operating in one direction at a time and each transferring data at up to 40 Gb/s. This provides a simultaneous total transfer rate of 80 Gb/s in each direction.

These lanes and transmitter-receiver pairs can be operated in several modes, including three for Thunderbolt 5 and USB4 2.0.

Single-lane USB4 is the same as the original USB4 already supported by all Apple silicon Macs, and in OWC’s superb Express 1M2 USB4 enclosure, and in practice its full 40 Gb/s comfortably outperforms Thunderbolt 3 for data transfers.

The first of the new high-speed Thunderbolt 5 and USB4 2.0 modes is known as Symmetric USB4, with both lanes bonded together to provide a total of 80 Gb/s in each direction. This is the mode that an external TB5 SSD operates in, to achieve claimed transfer rates of ‘up to’ 6 GB/s.

The other new mode is Asymmetric USB4. To achieve this, one of the Lanes has one of its transmitter-receiver pairs reversed. This provides a total of 120 Gb/s in one direction, and 40 Gb/s in the other, and is referred to in Thunderbolt 5 as Bandwidth Boost. This can be used upstream, from the peripheral to the Mac host, or more commonly downstream, where it could provide sufficient bandwidth to support high-res displays, for example.

Direct host connections

In practice, these modes should work transparently and to your advantage when connecting a peripheral direct to your Mac. If it’s a display, then the connection can switch to Asymmetric USB4 with its three transmitters in the host Mac, to deliver 120 Gb/s to that display. If it’s for storage or another device moving data in both directions, then Symmetric USB4 is good for 80 Gb/s in each direction, and should deliver those promised 6 GB/s read/write speeds.

Docks and hubs

When it comes to docks and hubs, though, there’s the potential for disappointment. Connect three high-res displays to your dock, and you want them to benefit from Asymmetric USB4 coming downstream from the host Mac. If the dock correctly switches to that mode for its connections to the displays, then it won’t work properly (if at all) if the connection between the dock and host is Symmetric USB4, as that will act as an 80 Gb/s bottleneck for the dock’s downstream 120 Gb/s.

Using a Thunderbolt 5 dock or hub is thus a great enabler, as it takes just one port on your Mac to feed up to three demanding peripherals, but it requires careful coordination of modes, and even then could fall short of your expectations.

Consider a TB5 Mac with a dock connected to one large high-res display, and a TB5 SSD. If modes are coordinated correctly, the Mac and dock will connect using Asymmetric mode to deliver 120 Gb/s downstream to the dock, then Asymmetric again from the dock to the display. But that leaves the SSD with what’s left over from that downstream bandwidth, although it still has 40 Gb/s on the return from the SSD through the dock to the Mac. That TB5 drive is then likely to perform as if it was an old USB4 drive, with perhaps half its normal read/write speeds. Of course, that’s still better than you’d get from a TB4 dock, but not what you paid for.

There’s also the potential for bugs and errors. I wonder if reported problems in getting three 6K displays working through a TB5 hub might come down to a failure to connect from Mac to dock in Asymmetric mode. What if the Mac and dock agree to operate in Asymmetric mode when the sole connected display doesn’t require that bandwidth, thus preventing an SSD from achieving an acceptable read speed?

Who needs TB5?

There will always be those who work with huge amounts of data and need as much speed as they can get. But for many, the most important use for Thunderbolt 5 is in the connection between Mac and dock or hub, as that’s the bottleneck that limits everyday performance. MacBook Pro and Mac mini models that now support TB5 come with three Thunderbolt 5 ports and one HDMI display port. You don’t have to indulge in excess to fill those up: add just one Studio Display and external storage for backups, and that Mac is down to its last Thunderbolt port.

As a result, Thunderbolt docks and hubs have proved popular, despite their bottleneck connection to the Mac. For many, that will be where Thunderbolt 5 proves its worth, provided it can get its modes straight and deliver the better performance we’re paying for.

When Apple launched its first M4 Macs just over a month ago, I was surprised that models with M4 Pro or Max chips offered Thunderbolt 5. Although there are still relatively few computers in use with support for TB5, and a dearth of peripherals, this article summarises early experience with this exotic new bus.

What TB5 peripherals are available?

As far as I’m aware, as of today there’s only one Thunderbolt 5 peripheral shipping in quantity, the Kensington SD5000T5 Thunderbolt 5 Triple Docking Station, with a total of three downstream TB5 ports. I’m just completing a full review of this, due to appear in MacFormat and MacLife magazines early next year.

SSDs have been announced by OWC in its Envoy Ultra, and Sabrent. The first of OWC’s have apparently started to ship, although they aren’t expected to become readily available until the New Year, when Sabrent’s are also expected.

OWC has also announced a TB5 hub, but that’s unlikely to appear until next year.

Several PCs are now available with TB5 support, although that seems to be fiddly to configure in Windows. Among those is the Razer Blade 18, which is even more expensive than a MacBook Pro with an M4 Max.

Other than those, there are lots of expensive TB5 cables, just precious little to connect to the other end.

Multiple displays

Many of those rushing to buy into TB5 are doing so because of its promised support for multiple displays. For example, the Kensington dock claims to support up to three 6K displays at 60 Hz with the M4 Max, and two with the M4 Pro. Although I have been unable to test those combinations, there are already reports that the M4 Max works well with three displays connected direct, but only two of those work when using the Kensington dock.

This has apparently taken Kensington and Apple by surprise, but until this has been addressed, I wouldn’t assume that you’ll be able to use all three displays attached to the dock.

Multiple SSDs

In early 2023, when TB4 hubs were becoming available, I wrote a whole series of articles here analysing their performance with a range of different SSDs. Links to those are given at the end. Those predated OWC’s superb Express 1M2 USB4 enclosure that now offers consistent and reliable performance for Apple silicon Macs, but not Intel models, which unfortunately lack support for USB4.

I have recently been revisiting SSD performance, both directly connected to my Mac mini M4 Pro, and working through the Kensington dock. Although some results are impressive, there are others that shock.

As shown in System Information, this dock connects to the host at 80 Gb/s, and to each USB4 drive at the expected 40 Gb/s.

On the bright side, 1M2 enclosures that return direct read/write speeds of 3.7/3.7 GB/s read almost as fast when attached through the dock, but their write speed drops to 2.3 GB/s, similar to many TB3 SSDs attached directly. You can even connect a USB4 drive to each of the dock’s three TB5 ports to benchmark them simultaneously, and get read/write results of 2.1/2.1 GB/s on each of them. That performance represents the maximum total data transfer capacity, matching claims of 6 GB/s made of TB5 SSDs, and equating to 80 Gb/s in TB5/USB4v2 symmetric mode.

Results from TB3 SSDs are more worrying. An award-winning certified Thunderbolt 3 SSD that achieves 2.9/2.2 GB/s read/write attached direct maintained a good read speed through the dock at 2.8 GB/s, but it almost ground to a halt during the write test, at 422 MB/s, that’s roughly the speed you’d expect from a basic SATA SSD.

You can read similar experiences during early testing of this dock for PC World.

For the time being, TB5 performs well with USB4 and directly connected TB3 SSDs, and the dock is a good solution for those wanting high-speed access to two or three USB4 SSDs in OWC Express 1M2 enclosures. The dock does have serious problems when writing to TB3 SSDs, though, where it may fall far short of expectations. Hopefully these problems will be resolved early next year.

Recommendations

• Although TB5 promises much, initial tests show that it currently has problems meeting that.
• Reports indicate that it may not yet support M4 Max chips driving three 6K displays at 60 Hz from a TB5 dock.
• Performance claimed for TB5 SSDs has not yet been confirmed in independent tests.
• Performance of TB3 SSDs attached to a TB5 dock demonstrates some very poor write speeds.
• Performance of USB4 SSDs attached to a TB5 dock demonstrates better and more consistent results, although their write speed also falls.
• TB5 cables and peripherals are expensive.
• Thunderbolt 5 is still at an experimental stage, and may take some time before it realises its potential.

Thunderbolt performance and TB4 hubs

General hub performance
Write speed throttling
How faster SSDs can impair performance of slower ones
Three SSDs on one hub
Getting best performance from Thunderbolt on Apple silicon Macs: a practical guide

Testing with Stibium

When using the ‘gold standard’ method of testing storage using my free Stibium, you don’t normally need to restart the Mac between write and read speed measurements. This has changed with the Mac mini M4 Pro, at least. If you go straight on to measure read speeds, results will be bogus because of what appears to be extensive caching of the files written during the previous write test. That results in absurdly high read speeds of more than 6 GB/s in most cases. This is surprising, as a total of just over 53 GB of files are written during the full write test, which seems far more than macOS should ever cache successfully!

For these tests on external SSDs, I therefore quit Stibium after measuring write speed, unmount the volume tested, remount it in Disk Utility, and open Stibium again to perform the read tests. This apparently clears caches reliably, and read speeds are consistent and in accord with those expected.

Interests

I bought my own Mac mini M4 Pro, Kensington SD5000T5 Thunderbolt 5 Triple Docking Station, and all the OWC Express 1M2 enclosures and their SSDs, at their regular retail prices. The only product tested here that has been provided by a manufacturer is, rather sadly, the TB3 SSD.

In the light of recent news, you might now be wondering whether you can afford to wait until next year in the hope that Apple then releases the M4 Mac of your dreams. To help guide you in your decision-making, this article explains what chip options are available in this month’s new M4 models, and how to choose between them.

CPU core types

Intel CPUs in modern Macs have several cores, all of them identical. Whether your Mac is running a background task like indexing for Spotlight, or running code for a time-critical user task, code is run across any of the available cores. In an Apple silicon chip like those in the M4 family, background tasks are normally constrained to efficiency (E) cores, leaving the performance (P) cores for your apps and other pressing user tasks. This brings significant energy economy for background tasks, and keeps your Mac more responsive to your demands.

Some tasks are normally constrained to run only on E cores. These include scheduled background tasks like Spotlight indexing, Time Machine backups, and some encoding of media. Game Mode is perhaps a more surprising E core user, as explained below.

Most user tasks are run preferentially on P cores, when they’re available. When there are more high-priority threads to be run than there are available P cores, then macOS will normally send them to be run on E cores instead. This also applies to threads running a Virtual Machine (VM) using lightweight virtualisation, whose threads will be preferentially scheduled on P cores when they’re available, even when code being run in the VM would normally be allocated to E cores.

macOS also controls the clock speed or frequency of cores. For background tasks running on E cores, their frequency is normally held relatively low, for best energy efficiency. When high-priority threads overspill onto E cores, they’re normally run at higher frequency, which is less energy-efficient but brings their performance closer to that of a P core. macOS goes to great lengths to schedule threads and control core frequencies to strike the best balance between energy efficiency and performance.

Unfortunately, it’s normally hard to see effects of frequency in apps like Activity Monitor. Its CPU % figures only show the percentage of cycles that are used for processing, and make no allowance for core frequency. It will therefore show a background thread running at low frequency but 100%, the same as a thread overspilt from P cores running at the maximum frequency of that E core. So when you see Spotlight indexing apparently taking 200% of CPU % on your Mac’s E cores, that might only be a small fraction of their maximum capacity if they were running at maximum frequency.

There are no differences between chips in the M4 family when it comes to each type of CPU core: each P core in a Base variant is the same as each in an M4 Pro or Max, with the same maximum frequency, and the same applies to E cores. macOS also allocates threads to different types of core using the same rules, and their frequencies are controlled the same as well. What differs between them is the number of each type of core, ranging from 4 P and 4 E in the 8-core variant of the Base M4, up to 12 P and 4 E in the 16-core variant of the M4 Max. Thus, their single-core benchmark results should be almost identical, although their multi-core results should vary according to the number of cores.

Game Mode

This mode is an exception to normal CPU and GPU core use, as it:

• gives preferential access to the E cores,
• gives highest priority access to the GPU,
• uses low-latency Bluetooth modes for input controllers and audio output.

However, my previous testing didn’t demonstrate that apps running in Game Mode were given exclusive access to E cores. But for gamers, it now appears that the more E cores, the better.

GPU cores

These are also used for tasks other than graphics, such as some of the more demanding calculations required for Machine Learning and AI. However, experience so far with Writing Tools in Sequoia 15.1 is that macOS currently offloads their heavy lifting to be run off-device in one of Apple’s dedicated servers. Although having plenty of GPU cores might well be valuable for non-graphics purposes in the future, for now there seems little advantage for many.

Thunderbolt 5

M4 Pro and Max, but not Base variants, come equipped with Thunderbolt ports that not only support Thunderbolt 3 and 4, but 5, as well as USB4. Thunderbolt 5 should effectively double the speed of connected TB5 SSDs, but to see that benefit, you’ll need to buy a TB5 SSD. Not only are they more expensive than TB3/4 models, but at present I know of only one range that’s due to ship this year. There will also be other peripherals with TB5 support, including at least one dock and one hub, although neither is available yet. The only TB5 accessories that are already available are cables, and even they are expensive.

TB5 also brings increased video bandwidth and support for DisplayPort 2.1, although even the M4 Max can’t make full use of that. If you’re looking to drive a combination of high-res displays, consult Apple’s Tech Specs carefully, as they’re complicated.

Although TB5 will become increasingly important over the next few years, TB3/4 and USB4 are far from dead yet and are supported by all M4 models.

Which M4 chip?

The table below summarises key figures for each of the variants in the M4 family that have now been released. It’s likely that next year Apple will release an Ultra, consisting of two M4 Max chips joined in tandem, in case you feel the burning desire for 24 P and 8 E cores.

Models available next week featuring each M4 chip are shown with green rectangles at the right.

There are two variants of the Base M4, one with 4P + 4E and 8 GPU cores, the same as Base variants in M1 to M3 families. There’s also the more capable variant, for the first time with 4P + 6E, which promises to be a better all-rounder, and when in Game Mode. It also has an extra couple of GPU cores.

The M4 Pro also comes in two variants, this time differing in the number of P cores, 8 or 10, and GPU cores, 16 or 20. Those overlap with the M4 Max, with 10 or 12 P cores and 32 or 40 GPU cores. Thus the gap between M4 Pro and Max isn’t as great as in the M3, with the GPUs in the M4 Max being aimed more at those working with high-res video, for instance. For more general use, there’s little difference between the 14-core Pro and Max.

Memory and storage

Chips in the M4 family also determine the maximum memory and internal SSD capacity. Apple has at last eliminated base models with only 8 GB of memory, and all now start with at least 16 GB. Base M4 chips are limited to a maximum of 32 GB, while the M4 Pro can go up to 64 GB, and the 16-core Max up to 128 GB, although in its 14-core variant, the Max is only available with 36 GB (I’m very grateful to Thomas for pointing this out below).

Unfortunately, Apple hasn’t increased the minimum size of internal SSD, which remains at 256 GB for some base models. Smaller SSDs may be cheaper, but they are also likely to have shorter lives, as under heavy use their small number of blocks will be erased for reuse more frequently. That may shorten their life expectancy to much less than the normal period of up to 10 years, as was seen in some of the first M1 models. This is more likely to occur when swap space is regularly used for virtual memory. I for one would have preferred 512 GB as a starting point.

While Base M4 chips come with SSDs up to 2 TB in size, both Pro and Max can be supplied with internal SSDs of up to 8 TB.

I hope this proves useful in guiding your decision.

Almost exactly a year after it released its first Macs featuring chips in the M3 family, Apple has replaced those with the first M4 models. Benchmarkers and core-counters are now busy trying to understand how these will change our Macs over the coming year or so. Before I reveal which model I have ordered, I’ll try to explain how these change the Mac landscape, concentrating primarily on CPU performance.

CPU cores

CPUs in the first two families, M1 and M2, came in two main designs, a Base variant with 4 Performance and 4 Efficiency cores, and a Pro/Max with 8 P and 2 or 4 E cores, that was doubled-up to make the Ultra something of a beast with its 16 P and 4 or 8 E cores. Last year Apple introduced three designs: the M3 Base has the same 4 P and 4 E CPU core configuration as in the M1 and M2 before it, but its Pro and Max variants are more distinct, with 6 P and 6 E in the Pro, and 10-12 P and 4 E cores in the Max. The M4 family changes this again, improving the Base and bringing the Pro and Max variants closer again.

As these are complicated by sub-variants and binned versions, I have brought the details together in a table.

I have set the core frequencies of the M4 in italics, as I have yet to confirm them, and there’s some confusion whether the maximum frequency of the P core is 4.3 or 4.4 GHz.

Each family of CPU cores has successively improved in-core performance, but the greatest changes are the result of increasing maximum core frequencies and core numbers. One crude but practical way to compare them is to total the maximum core frequencies in GHz for all the cores. Strictly speaking, this should take into account differences in processing units between P and E cores, but that also appears to have changed with each family, and is hard to compare. In the table, columns giving Σfn are therefore simply calculated as
(max P core frequency x P core count) + (max E core frequency x E core count)

Plotting those sum core frequencies by variant for each of the four families provides some interesting insights.

Here, each bar represents the sum core frequency of each full-spec variant. Those are grouped by the variant type (Base, Pro, Max, Ultra), and within those in family order (M1 purple, M2 pale blue, M3 dark blue, M4 red). Many trends are obvious, from the relatively low performance expected of the M1 family, except the Ultra, and the changes between families, for example the marked differences in the M4 Pro, and the M3 Max, against their immediate predecessors.

Sum core frequencies fall into three classes: 20-30, 35-45, and greater than 55 GHz. Three of the four chips in the M1 family are in the lowest of those, with only the M1 Ultra reaching the highest. The M4 is the first Base variant to reach the middle class, thanks in part to its additional two E cores. Two of the M4 variants (Pro and Max) have already reached the highest class, and any M4 Ultra would reach far above the top of the chart at 128 GHz.

Real-world performance will inevitably differ, and vary according to benchmark and app used for comparison. Although single-core performance has improved steadily, apps that only run in a single thread and can’t take advantage of multiple cores are likely to show little if any difference between variants in each family.

Game Mode is also of interest for those considering the two versions of the M4 Base, with 4 or 6 E cores. This is because that mode dedicates the E cores, together with the GPU, to the game being played. It’s likely that games that are more CPU-bound will perform significantly better on the six E cores of the 10-Core version of the iMac, which also comes with a 10-core GPU and four Thunderbolt 4 ports.

Memory and GPU

Memory bandwidth is also important, although for most apps we should assume that Apple’s engineers match that with likely demand from CPU, GPU, neural engine, and other parts of the chip. There will always be some threads that are more memory-bound, whose performance will be more dependant on memory bandwidth than CPU or GPU cores.

Although Apple claims successive improvements in GPU performance, the range in GPU cores has started at 8 and attained 32-40 in Max chips. Where the Max variants come into their own is support for multiple high-res displays, and challenging video editing and processing.

Thunderbolt and USB 3

The other big difference in these Macs is support for the new Thunderbolt 5 standard, available only in models with M4 Pro or M4 Max chips; Base variants still only support Thunderbolt 4. Although there are currently almost no Thunderbolt 5 peripherals available apart from an abundant supply of expensive cables, by the end of this year there should be at least one range of SSDs and one dock shipping.

As ever with claimed Thunderbolt performance, figures given don’t tell the whole story. Although both TB4 and USB4 claim ‘up to’ 40 Gb/s transfer rates, in practice external SSD performance is significantly different, with Thunderbolt topping out at about 3 GB/s and USB4 reaching up to 3.4 GB/s. In practice, TB5 won’t deliver the whole of its claimed maximum of 120 Gb/s to a single storage device, and current reports are that will only achieve disk transfers at 6 GB/s, or twice TB4. However, in use that’s close to the expected performance of internal SSDs in Apple silicon Macs, and should make booting from a TB5 external SSD almost indistinguishable in terms of speed.

As far as external ports go, this widens the gap between the M4 Pro Mac mini’s three TB5 ports, which should now deliver 3.4 GB/s over USB4 or 6 GB/s over TB5, and its two USB-C ports that are still restricted to USB 3.2 Gen 2 at 10 Gb/s, equating to 1 GB/s, the same as in M1 models from four years ago.

My choice

With a couple of T2 Macs and a MacBook Pro M3 Pro, I’ve been looking to replace my original Mac Studio M1 Max. As it looks likely that an M4 version of the Studio won’t be announced until well into next year, I’m taking the opportunity to shrink its already modest size to that of a new Mac mini. What better choice than an M4 Pro with 10 P and 4 E cores and a 20-core GPU, and the optional 10 Gb Ethernet? I seldom use the fourth Thunderbolt port on the Studio, and have already ordered a Kensington dock to deliver three TB5 ports from one on the Mac, and I’m sure it will drive my Studio Display every bit as well as the Studio has done.

If you have also been tempted by one of the new Mac minis, I was astonished to discover that three-year AppleCare+ for it costs less than £100, that’s two-thirds of the price that I pay each year for AppleCare+ on my MacBook Pro.

I look forward to diving deep into both my new Mac and Thunderbolt 5 in the coming weeks.