In your iSCSI post you're getting around 55-60 MBps of throughput, which is about 17 times faster than your initial NFS post.
There can be multiple reasons for poor NFS performance, from my experience most of the time they are NFS vendor / implementation (FreeNAS in your case) related so vendor documentation and best practices should be the first place to look at. I'm sure there is someone out there who was able to get excellent results with FreeNas as well.
WBR
Imants