SATA III is gigabit, so the max speed is actually 600MB/s.
My mistake, though still, a 4tb transfer should take less than 2hr at 5Gb/s (IN THEORY) Thank you @Max_P for pointing this out a second time elsewhere: 6Gb/s is what the sata 3 interface is capable of, NOT what the DRIVE is capable of. The marketing material for this drive has clearly psyched me out, the actual transfer speed is 210Mb/s
The filesystem is EXT4 and shared as a SMB… OMV has a fair amount of ram allocated to it, like 16gb or something gratuitous. I’m guessing the way rsync does it’s transfers is the culprit, and I honestly can’t complain because the integrity of the transfer is crucial.