I think with a lot of centre halves it is not their top speed that is the problem, it is the acceleration, the amount of time needed to get to top speed. They are some of the biggest lads on a football pitch, and often need twice as long as shorter players to get to top speed.
I don't see Evans as slow, but i think he could work on his off the mark acceleration. At a club like Utd whose defenders are often high up the pitch, recovery pace can be the difference between being a very good defender, or a great one.
Like it or not, lack of pace is a weakness that can be exploited as we saw when Torres targeted Vidic, rather than Rio a few years ago. That was the one advantage Torres had over Vidic and he used it to good effect. It is not absolutely imperative for a CB to have good pace, but it certainly helps.
Rio is a good example, only 2 or 3 years ago when he had that fantastic recovery pace, he was the best CB on the planet. Now he has lost that turn of pace, he looks much more vulnerable.