Update March 30, 2020: Perhaps this is exacerbated by changes in macOS Catalina, but we have found that APFS performance on the slower 5400RPM HDDs to be unacceptably poor. Stall analysis indicates that APFS gets bogged down when renaming files, especially in folders with many other files.
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.