FreeNAS NFS  dataset set  sync=disabled

zfs set sync=disabled tank/dataset 

Need to remeber have data risk.

https://www.ateamsystems.com/tech-blog/solved-performance-issues-with-freebsd-zfs-backed-esxi-storage-over-nfs/

http://christopher-technicalmusings.blogspot.tw/2011/06/speeding-up-freebsds-nfs-on-zfs-for-esx.html