[CentOS] nfs (or tcp or scheduler) changes between centos 5 and 6?

>> You may want to look at NFSometer and see if it can help.
> 
> Haven't seen that, will definitely give it a try!

Try "nfsstat -cn" on the clients to see if any particular NFS operations
occur more or less frequently on the C6 systems.

Also look at the "lookupcache" option found in "man nfs":

lookupcache=mode
Specifies how the kernel manages its cache of directory entries for a
given mount point. mode can be one of all, none, pos, or positive.  This
option is supported in kernels 2.6.28 and later.
(there is more text in the man page)

Since C5 came with 2.6.18 and C6 with 2.6.32 this might have something
to do with it.

Regards,
  Dennis