Should I format my SoftRAID volume as APFS or HFS+?
APFS is Apple’s first new file system in over 30 years. It offers a host of features not found in HFS+, including native encryption, copy-on-write, snapshots, and greatly improved volume stability. We recommend using APFS whenever you are creating a volume which only uses SSDs as this allows you to take advantage of these great features.
Unfortunately, one of these features, copy-on-write, can result in a volume that quickly becomes unusably slow. This can happen after just a few weeks of use on any APFS volume which uses HDDs. The slowdown will occur on any file which is written after being duplicated with the copy-on-write feature. Copy-on-write is used anytime: a file is copied or duplicated in the Finder and where both copies are on the same volume, when a snapshot of a volume is created or whenever any of the most popular backup utilities are used to back up a volume. For this reason, we do not recommend using APFS on volumes containing HDDs unless you are using the volume for a Time Machine backup.
Time Machine volumes must be formatted as APFS if you are running macOS 11 or later. This includes SoftRAID volumes which contain HDDs.
For a more detailed description of why the copy-on-write feature of APFS results in slow performance with HDDs, check out this video starting at the 3:20 mark.