Keith - Just a few thoughts you may have entertained…

In that it's a network file system, network throughput may be a factor, where 
gigabit ethernet and jumbo frames may help. Network statistics may reveal 
periodic packet losses and retransmissions which slow things down: the 
'nfsstat' command may illuminate issues. Various NFS performance guides may 
help, such as http://nfs.sourceforge.net/nfs-howto/ar01s05.html.

The file system architecture may be a contributor, depending upon file system 
type (ext2, ext3, ext4) as well as RAID configuration.  File system 
architectures with flat directory data structures and high populations can be a 
drag.  (File system design evolution has made for performance improvements.)  
Try some file system traversals, natively on the hosting system and then 
comparatively over NFS, to see if there's an issue there. Heavy file system 
activity by applications may cause contention, and can be more problematic if 
that access is over NFS as well.  You might also try traversals within various 
file system sections to possibly identify a sluggish area there there are very 
large directory populations or heavy activity.

Review your TSM accounting records or ANE session summary records in the 
Activity Log to get numbers on what's most contributing to TSM database 
activity and thus growth therein which may be slowing backups.

Note that excluding directories from backups is far more efficient than trying 
to exclude by all files in the directory.

     Richard Sims