I think the answer may be CPU clock speed, but basically the only way I can answer this question really is with a weird scenario which I thought about while on holiday..
Lets say there is a raspberry pi on the side of the road, counting how many cars have passed by having a focused IR light or laser stream being broken every time a car (or any opaque mass), passes by.
If the code reads something like: While lightNotBroken (): Do nothing Else: carCount+=1
The code is happily repeating itself really fast, great. But surely there is a theoretical speed (unrealistically fast), which if met, could cheat the program and not get counted as a car because the program wasn't executed fast enough to notice the broken stream of light..
What determines this speed?