The most improved defense is in Miami, where the Heat are holding teams to a ridiculously low 87.9 points per 100 possessions.
Three seasons ago, the Celtics had the best defense of the last 20 seasons, allowing 8.6 points per 100 possessions fewer than the league average. Miami isn't going to hold its opponents to 14.6 points fewer than the league average all season, but that five-game mark is impressive. In 2007-08, the Celtics had just three five-game stretches where they held their opponents under 88 points per 100 possessions.
Boston's best defensive five-game stretch came right after Thanksgiving that season, when the Celtics held five opponents to 86.1 points per 100 possessions. They were actually more dominant to start last season, when they were holding their opponents to 85.2 points per 100 possessions after five games and 85.4 after six.
The Heat would have to have another dominant defensive performance in New Orleans on Friday to match that six-game start, but for them to come close in their first five games together (with two of the games against strong offensive teams like Boston and Orlando) is pretty scary.