I think there's some truth in both sides. There are several possessions per game when the Celtics have the ability to push the pace, but for whatever reason (bad habits, personnel on the floor, game management, fatigue), do not.
I don't think it has quite as much to do with made or missed baskets. I think there is plenty of opportunity to push the "pace" even in the half court (pace is a tricky word because it's been used to describe shots taken, but I believe pace should more accurately reflect ball movement, or how much you make the defense move).
Just because a team is getting back on defense does not mean their defense is set and in position. There is still plenty of opportunity in transition to dictate the terms of the possession even when the defense is ahead of the ball. However, with the current Celtics squad I think this is largely a personnel matter. For instance, ideally the ball gets advanced via pass up the court to speed things up. However, Rondo often gets the ball around 3/4 or almost full court, especially when he's getting the rebound. He may have an opportunity to advance it to Avery Bradley or Evan Turner around half court, which would create a more up-tempo attack. Personally, I don't like Bradley or Turner taking it from half court with the defense ahead of them and Rondo still in the back court. In these scenarios I'd rather just have Rondo take the ball the extra 1/4 length of the court than advance the ball to one of these players (it's different if one of these players is ahead of the defense, or if they are further than half court).
That being said, where I would really like to see improvement is in the half court. Sometimes ball movement is great, but there are many times where 3-5 seconds are wasted with the ball mostly stationary behind the 3 pt line. I'd like to see better use of this time to increase half court pace/ball movement. They've been getting some really great shots for Evan Turner lately, but I feel like they aren't good about transition back to moving the ball around to other players after that. I think that's one reason the fourth quarters get stagnant.