Nervous System
The sloth's nervous system has evolved so much that even when a sloth is in a panic because its young is being preyed upon, it still manages to move very slowly when it tries to rescue its own young. Their slow motion has nothing to do with diet. Koalas and leaf eating monkeys can move very quickly if they are trying to escape a predator. It is just that there are two different strategies in predator avoidance. One is to move quickly and the other one is to move slowly.
http://www.youtube.com/watch?v=1ErqVDccwH4
http://www.youtube.com/watch?v=1ErqVDccwH4