It’s commonly claimed that software encryption, as used in APFS Encrypted format, incurs negligible overhead. The last time I looked at that was with Thunderbolt 3 SSDs connected to a Mac Studio M1 Max, when I found that varied according to the SSD. One of the three I tested then did show significant reductions in encrypted write speed, from 2.2 to 1.8 GB/s, but the fastest showed no change from its unencrypted write speed of 2.8 GB/s. This article reports new test results from a Mac mini M4 Pro with faster SSDs, one Thunderbolt 5 and the other USB4, and adds data for computing SHA256 hashes.

These are of particular interest, as not only are the unencrypted transfer speeds for both SSDs significantly higher than Thunderbolt 3, but the host has significantly faster CPU cores.

Two sets of measurements were made on each of the two SSDs:

• Stibium 1.2 running on macOS 15.5 Sequoia was used to measure read and write speeds over randomised sequences of a total of 53 GB in 160 files of 2 MB to 2 GB individual size.
• Stibium was used to measure the single file read speed of a 16.8 GB IPSW file, and Dintch was used to measure the time taken to stream the file in and compute its SHA256 digest, using CryptoKit.

Read and write speeds

Results of the first series of tests showed both SSDs performed as expected when using plain APFS, with read and write speeds of 5.3 GB/s for TB5, and 3.7 GB/s for USB4.

Small reductions in read speed were seen in both SSDs when using APFS Encrypted, to about 98% and 95% of their unencrypted read speed. Although there was a similar small reduction in write speed for USB4, to 97%, that seen in the Thunderbolt 5 SSD was greater, with a fall from 5.3 to 4.7 GB/s (89%). Both sets of tests were repeated for that SSD, allowing ample time for the SLC cache to be emptied after each set of write tests, and results remained essentially the same.

Although write speed to APFS Encrypted for this Thunderbolt 5 SSD remained well above that for USB4, encryption brought a reduction in speed of just over 10%, more than I had anticipated.

Hash computation

SHA256 and SHA512 digests are now used to check file data integrity. Both are computationally intensive, and I have previously reported that reading files of substantial size and computing their digests using CryptoKit proceeds at about 3 GB/s for files stored on the fast internal SSD of a Mac mini M4 Pro.

With the Thunderbolt 5 SSD, a plain file of 16.8 GB was read at 6.5 GB/s, and encrypted at 4.7 GB/s. SHA256 digest computation was performed at 2.6 GB/s from plain APFS, and 2.2 GB/s from APFS Encrypted, both well below that from the internal SSD, and less than half the speed of just reading the file.

Although the USB4 SSD was inevitably slower on the read tests, at 3.8 GB/s, encryption had little effect, at 3.7 GB/s. SHA256 digest computation was, if anything, faster than with Thunderbolt 5, at 2.8 GB/s plain, and 2.7 GB/s encrypted.

Conclusions

Although there may well be differences with other Thunderbolt 5 and USB4 SSDs, and more extensive results would be helpful:

• Whether plain or encrypted APFS, Thunderbolt 5 SSDs are substantially faster than USB4.
• Encryption can result in significantly lower write speeds on some Thunderbolt 5 SSDs.
• Otherwise, encryption has only small effects on read and write speeds.
• Computation of SHA256 digests is significantly slower than encryption, and ranges between 2.2-2.8 GB/s on larger files.
• This suggests that, even in faster M4 chips, CPU performance limits the speed of software encryption, and even more so for SHA256 digest computation.