I did some research on this in April last year on an old, old kernel.
One of the codepaths I flagged:
There were small gains to be had by reordering the sync of the parent and
child syncs where the two inodes were in the same cluster. The larger
problem seemed to be that we're not treating the log as stable storage.
By calling write_inode_now we've written the changes to the log first
and then gone and also written them out to the inode.
nfsd_create, nfsd_link, and nfsd_setattr all do this (or do in the old
kernel I'm looking at). I have a patchset that changes
this to an fsync so we force the log and call it good. I'll be happy to
dust it off if someone hasn't already addressed this situation.