I know for a fact that Pidgeot can fly at mach 2, so that's something. I can get some stats for comparison purposes, but that might count as game mechanics. But then again:
Pidgeot's base speed stat is 91. It travels at mach 2.
Alakazam's base speed stat is 120. Its brain can outperform a supercomputer.
There are tons of other pokemon that are too fast to be seen, but aren't given concrete numbers.
Anyway, Mewtwo's base speed stat is 130. He's pretty darn fast. Another interesting fact I discovered is that, according to the games, Mewtwo is god level. His stat total is 680. The only pokemon with a higher total is Arceus himself, at 720. There are a few other pokemon with a total of 680. They are Ho-Oh, Lugia, Rayquaza, Dialga, Palkia, and Giratina. All of them are considered gods or godlikes.
I find it funny that Mewtwo can hold his own against the creators of Time and Space (like, you know...Chaos Control.) And he's faster than them, too.