Well, thats bad!
I'm not able to offer much support but can you tell about your setup: how many disks do you use for data? raw or vmdk? In esxi, does it show high cpu usage even when idle?
I found the problem: it was made by my multiple tests on this VM.
vSphere Client told me to consolidate my snapshots (even if i had no more snapshot); After consolidate, transfers speed is identical to freenas !