It might be the processor speed that's an issue. Try doing basic slow processor troubleshooting and see if that improves
Ah yes make sure to hit the turbo button on your computer before using any usb3 devices.
First restart, then try any other USB3 devices. Are you copying the exact same files to/from? Writing a file TO the drive will have a different speed than copying/moving it FROM the drive, and if it's not the exact same file the speed can vary.