All cores in CPU are 100% utilitized when copying file over network
That's mainly the unfortunate tale of thermally inept design. The kernel_task uses up so much because the temperature has risen too high. Install a sensor app and confirm, install Intel CPU PowerGadget to confirm again. Widespread and common problem.

Thunderbolt chips get hot and are inadequately connected to the cooling system and so the system tries to slow things down by occupying the CPU with meaningless cycle sucking to prevent a halt and catch fire situation.

To mitigate this, you might try to disconnect other peripherals, install a fan control software that is set to max rpm, use external cooling solutions (stands, pads, fans, AC), stop all other processes causing calculations that translate into heat. If you are daring and think you know better than Apple's 'designers' on how much heat your machine should be able to stand, you may also override this with some hacking. (This is not recommended.)