External USB drive is limited by the speed of the USB device. USB is UP TO 640 MBps
USB port used for that device will determine speed. So a USB 2 port will only work max 60MBps.
Thing to remember those are the max speeds.
Moving that amount of data is not particularly hard work for your system, or any modern system these days. So no it will not ramp up the CPU core's at all, or very much. The IMC will work, internal memory controller, as it is handling the short term storage during the transfer. Some other bits of the board hardware will have 'work' handed over to that component like the USB controller...
So nothing to worry about.
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