Nupz, it was never explicitly said that Mewtwo in the movie knew Extreme Speed.
My guess is that according to the stats, Mewtwo (speed = 130, it's 140 for Mega Mewtwo Y (the one which was featured in the movie), is much faster than Genesect (speed = 99).
As for Red Genesect... Well, ESpeed is a physical move in the game. So, I'm not really sure how to compare the speed stats if a Pokemon is using the move. But judging by their speed stat difference, I guess it's fair to say that Mewtwo was able to keep up with Red Genesect's ES without using the move.