Probably a stupid suggestion, but are all the drivers up to date? Maybe worth completely removing the USB drivers and re-installing?
this sounds like a case of cheap leads and cheap expansion card combos. Unfortunately PC cables are unlike HDMI and stuff and to get anywhere near optimum speeds you need to make sure that everything from PSU to cards in PCIE 8/16 slots and the associated cable are all working in harmony. First thing I would do would be to go on Amazon, get a decent brand USB PCIE adaptor and the official link cable because if you keep going on your present route its going to cost you more in the long run anyway. Then if this stuff doesn’t work you can send it back to Amazon for a no quibble refund anyway. Start off with the cable first then work back to the adapter. The fact that people are getting only 2.3 Gbps on the average priced cables compared to 3 on the official cable and are then suffering stuttering tells you all you need to know.