My APFS-formatted rotational disks have always felt slower than when they were HFS+ formatted. The speed of copying files to them felt about the same, but slogging through folders in the Finder was taking a lot longer. At first I shrugged it off to the filesystem being new; "It just needs some tuning, it will come along." But that performance hasn't come along, and after running some tests and collecting a lot more data, I'm convinced that Apple made a fundamental design choice in APFS that makes its performance worse than HFS+ on rotational disks. Performance starts out at a significant deficit to HFS+ (OS X Extended) and declines linearly as you add files to the volume.
The rest of this article is fairly technical, here are the key takeaways:
- Enumerating an APFS filesystem on a traditional HDD (rotational disk) will take 3-20X longer than HFS+ on the same hardware.
- This performance difference is most noticeable on a macOS startup disk that is (or includes) a rotational disk.
- If Apple doesn't make some concessions in the APFS filesystem to accommodate the slower seek performance of HDD devices, then a rotational device will never be able to provide acceptable performance as a production macOS startup disk.