Bottleneck

A bottleneck is the single slowest step in a process that limits how much work can get through the whole thing, no matter how fast the other steps are. Like the narrow neck of a bottle, it sets the pace: widen everything else and nothing improves until you widen the neck.

It is like one slow checkout in a busy shop, where the queue builds no matter how quickly the shelves are stocked. Finding the bottleneck matters because effort spent speeding up any other step is wasted, while a small improvement at the real constraint speeds up everything downstream.