Just a few thoughts:
3500KBps (~3.5MBps=~28Mbps) is slow, even for a 100Mbps link. Gigabit connection normally should be able to produce ~80MBps.
Aparently there is a bottleneck somewhere down the path. You'll have to find it.
- check link speeds on all involved ports
- check logs, disk, CPU and network statistics on your NFS server
- check dropped packet / error counters on vSwitch and pSwitch
- check latency counters in vSphere
how far is the NFS box from your host? how many hops are there along the path? are there any routers in between?
if the speed is dropping after certain period of activity, might be that some buffer or queue somewhere is filling up, could be some sort of a "cache" on NFS (I'm not a FreeNAS expert, in case you're not either, go out and find one), or buffers on pSwitch.
and don't forget about general troubleshooting best practices - benchmark, one documented change at a time and then benchmark again.
hope this helps
WBR
Imants