Make sure a firewall rule isn’t blocking NFS. If NFS is running on the server and clients _can_ mount, but it’s just really slow, then things get a little hairly. You can’t just look for a problem on a client or a fix a misconfigured server. You’ll have to look at the whole ball of wax… If MTU mismatch doesn’t seem to be a problem, try going the other way and increasing the MTU size. Use the traceroute command to look for unexpected routing hops or delays.

Opportunistic locking is part of the Windows client file caching mechanism. Samba implements opportunistic locking as a server-side component of the client caching mechanism. Samba/CIFS doesn’t play nice with NFS, so if you’re in a mixed environment where some windows machines access and modify the same files that Linux or Solaris touch through NFS, then disable the oplocks. This is important for things like database files to avoid corruption!