Lower case mb? m is usually milli but OK, we are talking computers so I'll correct that the M or millions.
The b is bits but 30 megabits a second would be slow indeed. Maybe you meant B for Bytes?
The USB HDD can get slow where there are drive issues. You'll see this on surface problems, or the drive will disconnect without notice or lockup the PC.
I have windows 10 and in device manager under usb it says Asmedia usb3.0 extensible host controller. I used flashbench to test an external usb hard drive on several usb ports. Each time it gives the read and write speeds at 35 mb/sec.
What would I need to do to get usb 3.0 speed?