Go read "The case for the 40-minute game" (http://grantland.com/features/the-case-40-minute-nba-game/) to understand why some basketball fans think NBA games are too long.
A couple of quotes to give you a flavour:
"Those 48 minutes stretch longer than ever now, [with] approximately 967 timeouts that interrupt any given nationally televised game."
"A three-hour NBA game goes against everything basketball is supposed to be — a fast-paced ballet of whirring athleticism, back-and-forth action, and fantastic players looping around the floor in coordinated geometric patterns."
Maybe some of this thinking is behind the 40 minute games in Summer League?