Familiarity, Contempt and Regular Season Only Scoring Plays
One example of the kinds of little things that can get a team a ton of extra good looks in the regular season is the Hawks' secondary break action:
In a playoff series, and certainly by the second game of a series, a decently coached team will have this action scouted and will have reminded their bigs to not automatically sink into the paint on ball reversal. And since the defenders will have to worry about the Hawks and only the Hawks, they will be better able to execute defensively. Thus the game "slows down" because one of Atlanta's primary weapons for early offense has been removed.