The way I like to think of it is like a road trip.
Imagine you had to drive from Toronto to Miami and you use google maps, you print out the route, and you know which way to... except, as you're driving along, you find that there's constrution on one of the roads and it's closed. What do you? Either you do something stupid like go through teh closed road or you find another route.
Same goes with performance. Sometimes certain slights just won't be sensible for a certain situation, so you have to use others.