The difference in speed is genetic. The medium cheetah can catch some antelopes, but not that many. The fast cheetah has ten cubs which survive to adulthood and breed themselves. The lice have evolved. That's how evolution works.
Evolution leads to more complexity and 'improvement'.
Natural selection is a process which slows down degeneration. A cheetah is supposed to run for 60 mph. Those cheetahs which degenerate, i.e. , can only run for less than 60 mph die, which implies that in general the capabilities increase. Of course not. If evolution were true, we would expect that cheetahs in a million years time will run for 200 mph.