Physics sleeping
A body whose motion drops below a threshold is pulled out of the simulation until something wakes it.
When a rigid body's linear and angular velocity stay below a threshold for a short while, the engine marks it sleeping and stops integrating it — no solver work, no collision response — until a moving body touches it or a force is applied. It's why a settled stack of boxes costs next to nothing.
The technique is tightly bound to islands: a whole island sleeps only once every body in it is quiet, and wakes as a unit. The trap is the threshold — set it too eagerly and objects freeze mid-motion; too loosely and the CPU grinds away on a nearly static scene.