I found another users test results using TB on the MOTU. They did not manage anything under 7ms at 44.1k.
https://www.gearslutz.com/board/12967785-post1613.html
It seems they just bridged the old f/w controller to TB for this unit so latency is pretty much the same the old f/w units.
So maybe you're doing something wrong in your testing. What i/o figures does Reaper actually report for 64 samples, 44.1k?