This got me curious and it seems that there is known APFS behaviors slowing things down on parallel I/O on same folders [1]

I would suspect that this is related to to the fact that reading/writing dirs have no atomic semantics in POSIX fs, and same directories are easily read/written simultaneously by unrelated processes, but surely is cause to edge cases bugs, especially in a world with backup system based on fs snapshots.

This is a sad state of affairs.

https://gregoryszorc.com/blog/2018/10/29/global-kernel-locks...