@lewisdawson1 said in Running RetroPi entirely on USB:

Is there a command i need to use for USB to access its full speed?

No really, just use the USB3 ports on the Pi4.
How are you powering up the disc - does it have its own power or is it just connected to the Pi ? Some USB3 controllers have issues with Linux - check this topic in the Raspberry PI forums and see if your disc/controller are affected by this problem.