I was dissappointed in the transfer speed of my USB 3.0 enclosure. It was transferring 75-90 MB/s with a 2TB 7200RPM Seagate in it. The same drive on internal SATA connection would peak at 190 and sustain in the 140s.
Compare that to me Toshiba Canvio 2TB 5900 (or is it 5400)RPM that will peak above 130MB/s and easily sustain about 110-120MB/s.
Same PC, same port. Maybe it's the chipset in the enclosure? Maybe a bad enclosure?