Which step limits the whole process?
Improving a fast step may do little when another step restricts the flow. If a packing station handles 20 orders an hour but inspection handles only 10, speeding packing to 30 cannot produce 30 inspected orders. The bottleneck sets the system's output under these simplified conditions.
Go deeper
Follow the slowest required stage
Consider a process with two required steps: packing and inspection. Packing can complete twenty orders an hour, while inspection can complete ten.
If work is available continuously, the process cannot sustainably finish more than ten inspected orders an hour. Extra packing capacity mainly creates a growing queue before inspection.
A bottleneck is the constraint currently limiting system output. Improving it can raise total capacity, but only until another step becomes the limiting stage.
The slowest-looking activity is not always the bottleneck. Demand, staffing schedules and shared equipment can change where the actual constraint sits. The useful question is which restriction prevents more completed work, rather than which worker appears busiest.