Tevez isn't slow. What he lacks is quick acceleration. Over a short distance he doesn't pick up much speed. He is like a 200 meter runner in that sense.
I said something different earlier...
I think that once he gets his head down he is pretty rapid and even over short distances, mainly when hassling other players, he is quick. When he has the ball it takes him a while to get going - being a playmaker he is often looking for a pass and whatnot so his initial thought isn't sprint or run, he likes to turn and keep the ball safe, an Argentinian trait actually.
You can see he isn't slow and could certainly beat a premiership defence running backwards ie ball over the top.