>>> [ ... ] xfs is very slow when deleting a large number of
>>> files. [ ... ]
Not quite -- it is nearly as fast as possible.
> Thanks. Yes, I know about these tricks like 'nobarrier'
> option, separate partition for metadata log, increased size
> for the log, more log buffers or atime disabled. But I was
> interested more in what is the real cause of this slowness.
Tricks like those indicate quite clearly the real cause.
To make it even more explicit, the real cause is that those who
complain tha XFS is slow at deleting lots of files have a storage
subsystem can only do a limited number of committed transactions
per second and XFS does not "cheat".