We don't play at a slow pace by design. We play slow because Rondo doesn't run constantly among other things. Doc is always shouting to push the ball. When we do, we're awesome. Our best performances, and by extension also Rondo's, is when he's pushing the ball for easy buckets.
Actually, the Celtics do play at a slow pace by design. Doc's preference to get back on defense instead of crashing the offensive boards is intended to make it a slower half-court game and prevent things from turning into an up-and-down transition game. Boston is supposed to score quick, easy buckets off of turnovers forced by the defense, but otherwise, the game is supposed to be a grind.
It's one thing to play carelessly, make ill advised passes from one half of the court to another, and not get back on defense. It's another to have your point-guard push the ball everytime he has the ball on his hands to look for easy opportunities. And if it's not there, then slow it down and look for a good shot.
Let me put it this way, the reason we aren't running more or pushing the ball more has nothing to do with a desire to play at a slower pace. This only happens when the team is being careless and being out of control, but one thing doesn't equate the other.