I.e. storagenode would delete much faster if unlink syscalls would complete faster. I’m not sure if parallelizing calls would help – there are 48 cores that are pretty much idle – it would depend on whether slow bits are under locks and have to be serialized.
The main reason we avoided parallelizing the thrashing or walking over the pieces, is can easily end up saturating the disk iops. Which in turn can make the node worse with regards to uploads and downloads.
This is also the reason that file walking was moved to a separate process, which can run at lower priority.
I am wondering whether ZFS is making things worse than compared to other file systems. I found a relevant issue when delete lots of files zfs system slow · Issue #13707 · openzfs/zfs · GitHub.