USB Keyboard dongle slows down retro CPU's cache [video]

I'm absolutely bewildered by this. Comments on the video are guessing that this is due to USB polling, because context switches on older CPUs weren't that great. But those are only guesses.

Here's a different USB related enigma I encountered. When you move your mouse pointer on the desktop, sometimes you can hear high pitched coil noise from your GPU, as long as you're moving the pointer. Sometimes it's pretty loud. I never understood why this happens...