What you see is that slow I/O (serial port/MIDI/DMX, Keyboard, Sound, network etc) uses buffers/interrupts. Faster processes (Disk/Screen) don't.
So what happens when a new font has to be cached, millions of cycles are waisted when the head of the harddisk seeks the right track (a small delay when printing to the screen). Under interrupt, the slow processes keep on running.
But who will finish the printing to the screen first? The pre-emptive system has to wait until the other interrupts are finished. The co_op system continues at ones (OK, I know, you can not see the difference it is late half a milion clock ticks but for time critical systems it can make a difference).