Question, did you try other USB ports?
If your have a lot of USB stuff connected it can hinder the throughput of the USB-port itself.
Many times USB ports are shared-bandwidth, either on the PCIe lanes other times the USB chip itself.
With devices like printers and mouses you will not notice such, but with high-bit-rate devices like the SDRplay it will cause troubles.
Programs like these: https://www.uwe-sieber.de/usbtreeview_e.html
Can help you find the problem.
Don't forget, 5M Sample-rate is a very big load on an USB-bus, if it's shared or not up to the task (happens more then you think), it will not perform at the upper limits.