To give a corresponding example, try moving the mouse while loading from floppy disk on Windows 95, ten years later. The mouse pointer judders!

This does not happen on a 1985 Amiga, even with a fraction of the CPU power, which it needs all of it to redraw UI damage as seen.

The reason is that the Amiga system prioritised input processing above normal tasks, it had preemptive multitasking where hardware timers triggered aCPU interrupt to let the kernel switch to a new timeslice, and of course reading a floppy disk was done with hardware interrupts and DMA transfers because the OS knew exactly what hardware it was dealing with, while PC compatibles couldn't guarantee those transfer modes (https://wiki.osdev.org/Floppy_Disk_Controller) so Windows 95 would inevitably handle floppy drives with polling.

Windows 95 was hamstrung by DOS compatibility. If you installed OS/2, you could have butter-smooth mouse cursor movement even while formatting a floppy disk and running a DOS (or even Windows!) VM in another window. And if you had a XGA card (which no one did) you had a hardware mouse cursor sprite, just like the Amiga (except only one boring sprite and no cool audio/video tricks)

The mouse pointer on Amiga was a sprite. So it flies above the bitmaps below, thence the 50fps frame rate.