Forensic Hardware: USB3.0 Cards For Tableau Writeblockers

Hello, We recently upgraded to the same Tableau T3589iu writeblockers and have been experiencing issues with the speed when imaging USB devices. We have fitted Akasa USB 3.0 cards with superspeed ports. The tableau is connected to the internal port on this card. Using FTK Imager, we get up to about 90mb/sec when imaging through SATA but this drops alot when imaging via USB. USB 3.0 devices are hitting speeds between USB 1.0 and USB 2.0 speeds. I am curious as to the speeds you are getting? Thanks