G5 5590, sharing USB3 hub with USB2 and USB3 devices - speed problem

I am trying to run a USB3 data video capture device on the G5 5590 laptop and suffering frame loss and latch up.

This is a known symptom if the device is used on a USB2 port - but all the ports on the G5 5590 are USB3 so shouldn't be a problem.

I have found the problem gets worse if I am also using a USB2 sound capture at the same time.

If I plug USB2 and USB3 devices into the laptop - do all the ports drop performance to USB2 levels?

Is there something else I can check for?