We've got a wierd issue recently whereby both our hosts seem to have completely lost their NFS performance - it literally falls off a cliff after a few hundred KB is transferred.
We're using QLogic 8242 CNA cards (two per Dell host with 5.1) and both died at around the same time. There's no issue with the NFS server (NetApp 3240) as other machines using it weren't affected, nor was anything else on the switch. It sounds like a dvSwitch issue, but I've moved one of the boxes NFS vmkernel to a standard vSwitch but it still has the same issue. On the other host, we've changed the cards and created a new dvSwitch for it and everything is up and running again.
We've logged it with VMWare, but support seems to be very slow, so thought I'd see if anyone here had any ideas?!