This is a wierd one & I've got to put it down to a BIOS firmware bug.   I disabled the three groups of USB 2 ports in the BIOS & the USB 3 ports are now working as they should. So I'm a happy bunny as 4 ports on the system is enough for my application (and there are always external hubs if I need more).
Hi folks   Hoping for a bit of help here.   Just acquired an Optiplex 9020 SFF for an application which requires fast USB ports. The chassis has 2 ports on the front panel & 2 on the rear which are marked "SS" (superspeed). But the transfer speed on these is around 35 Mbits/sec, the same as the other ports which are USB2.   I've updated the BIOS to the current version, checked there's no setting in it which would make the USB ports run slow & checked that I have the correct drivers installed.   Any help? I'm guessing an undocumented jumper on the system board or a hardware failure. If the latter then the darned thing will have to be returned.
Optiplex 9020 SFF USB3 ports work but no faster than USB2