Raid-Z2 SMB read speed ~66MB/s (slow?)

Despite being new to FreeNAS, over the past few months of use I have really come to appreciate how powerful it is as a storage operating system (in particular ZFS with adequate RAM!).

Drive Setup:
6x WD Red 3TB in Raid-Z2 (Half the drives have the 80.00 firmware, the others have 82.00)
4 connected via SATA2 onboard controller, 2 via SATA3 marvell controller (88SE9128 chipset)

Storage Pool settings:
Sync: Standard
Compression level: lz4
Share Type: Windows
Enable Atime: off...

Raid-Z2 SMB read speed ~66MB/s (slow?)