The Celtics' average pace — possessions per game — has been significantly higher than the NBA average this season. As of Jan 8, the Cs are averaging 98.0 possessions per game, 4th highest in the league. (League average = 95.7.)
According to Brad Stevens, this is by design. The Cs' general offensive game plan is to outhustle opponents, make em work harder than they want to, and capture easy points in transition as often as possible.
In this way, the Celts hope to counterbalance their relative dearth of raw offensive firepower, and win some games.