According to
http://www.expertreviews.co.uk/inter...viar-green-2tb the disk is quite slow with its' 5400 rpm, especially in transferring small files.
Your test with
tmpfs seems to rule out your ZFS setup as cause, so you could try with another disk.
Another plausible cause could be the infamous 4K sector alignment issue.
What do
gpart and
diskinfo report about the disk?