[Mark McLeod]

The fast loop is variable because different things will happen each time through. Sometimes it'll be executing more instructions than average, other times fewer instructions than average. For instance, if you have a few interrupts from clock timers or external sources, or if an event happens that your code decides to run a longer calculation on. Any of the conditional statements you have in your code mean that sometimes you'll go down one branch and at other times you'll choose a different branch through the code. This doesn't even count the timing changes due to printfs or IFI library code. All these code variations add up to variable time through the fast loop. That's why most of us use timers or the more dependable slow loop.