I remember back in the Amiga days, the processes for the GUI would be totally separate from the program logic. Add the pre-emptive multitasking and you would have GUI elements that would respond instantly always, even when the rest of your program was hung! Of course, the button callbacks wouldn't fire, but the button would appear to press, you could move windows, etc. Did a lot to reduce the perception of lag, whilst obviously introducing quite a few quirks of its own.