slow usb transfer

USB-3 is capable for 5,000 M bits/sec, or about 500 M Bytes/sec.

But using USB-3 depends on making certain that every cable and adapter has all nine conductors, and is of high quality. Often, USB-3 adapters and cable have a Blue insert to indicate that carry nine conductors instead of USB-2 four conductors.

Any other USB-2 device on the same USB Bus can cause all the devices on that bus to downshift to USB-2 speeds, 480 M bits/sec or 48 M Bytes/sec, about the speed of a rotating magnetic drive.

USB thumb drives can be very slow. Even thumb drives that can transfer data at USB-3 speeds, may not be able to store or read data at those speeds for more than a moment.

Restart and retry.

If the drives a thumb type, that's likely the problem. They are quite slow by default.