Let me add:
Celtics currently rank 4th in team defensive rating, while ranking 5th in pace.
And they are doing this with Sully, arguably one of the slower players in the league, and solid bigs like Johnson, Lee or Olynyk - none of them is particularly quick or athletic .
I think it's safe to say, that we could do a much better job on defense with bigs like Willie, Kosta and Cuz, even if we keep playing at a high pace. Of course our perimeter defenders don't have the same quality like Bradley, Smart or Crowder, but we don't need to be elite on D. We just need to be decent to win games, because we are actually pretty talented offensively.
Now what's the difference between us and the Celtics? Coaching might have something to do with it (I would ship Karl to Boston in a heartbeat for Stevens), but I think it's mainly the mentality of the players.
We have all the tools to apply a good amount of defensive pressure on the perimeter, but we just lack the "this guy will not score on my watch"-mentality, that nearly every Celtic, even IT, brings to the floor.
So yes JdBraver is right - it's mainly effort!