Slow FireWire 800 Port

I'm rocking a 2011 iMac 2.5 with 16GB of RAM, running 10.11. I've recently come to the realization that my firewire drives are not running as fast as they have on other macs. I have two daisy chained drives, a LaCie 2Big Triple and a G-Drive, both using Firewire 800. On both drives, I'm getting a maximum of 60MB/s read, 40MB/s write--tested using BlackMagic & AJA. I've tried all the logical steps: swapping different cables, testing the drives individually [instead of daisy chained], reformatting them and testing again, all with the same result. I even tried turning FileVault off on my internal drive, though I can't imagine why that would make a difference. These same drives have been tested in the same configuration with a 2011 MacBook Pro 13", also running El Capitan, and I've seen regular speeds of 75MB/s read, 70MB/s write.

Anyone have any ideas? I'd greatly appreciate any help you can offer.

Once again, here are the technical specs of the equipment involved:

• Mid 2011 iMac 21.5", 2.5GHz i5 Quad, 16GB RAM, 500GB HDD. Running OS X 10.11.4 El Capitan.
• G-Drive 5th Generation, with 2TB Hitachi 7200rpm drive. Formatted as HFS+, connected via FW800
• LaCie 2Big Triple, containing two 1TB drives in a striped RAID.