The level transition has a timetable
In the Mario game described here, a level does not load the instant the player reaches its exit. The transition can happen only at certain frame intervals: every 21 frames. At 60 frames per second, that interval is 0.35 seconds—short enough to feel instantaneous to most players, but long enough to matter in a speedrun.
A frame is one snapshot of the game’s action. The game advances through these snapshots in sequence, and its loading window acts like a scheduled departure: arrive just before it, and the next level begins at the next opportunity; miss it, and the runner may have to wait for the following one.
Why runners time their arrival
Speedrunners optimize more than the path through a level. They also try to make their movement line up with the transition schedule. A route that reaches the exit a fraction of a second earlier can be slower overall if it arrives just after a loading window and has to sit through another 21-frame cycle.
That makes the final approach a timing problem. The runner needs to cover the level quickly, but also arrive at the right moment—not merely touch the exit as soon as possible. The “invisible bus” is the next loading window: nobody sees it, but missing it can cost a measurable slice of the run.
