Simulation island
A group of bodies that touch or constrain one another and so must be solved together — separate islands are solved, and put to sleep, on their own.
If two boxes have nothing to do with each other, there is no reason to solve their contacts in the same sweep. The engine therefore groups bodies into islands: connected components of the contact graph. Each island is solved independently, which also makes it easy to spread the work across threads.
The big win is sleeping. Once every body in an island has stood still long enough, the whole island can be marked asleep and skipped entirely until something wakes it. A town full of untouched crates then costs almost nothing until the player runs into them.