Believe it or not, it is harder to play slow and stay in time. I know it sounds crazy but the reason is most people a tempted to hit the next note just that little bit too soon. I think anyone with enough practise could play resonable fast and clean, and accurately, but ask that same person to play a solo involving about twenty notes and which relays on pauses, accents, etc. I think they may struggle for a while. It may sound a little crazy, but it's true.