Process bottleneck
A process bottleneck is the step in a workflow with the lowest throughput, the constraint that limits the entire process's output. Identifying the bottleneck is the prerequisite to any throughput-improvement work, because non-bottleneck improvements don't move the system-level throughput.
Bottlenecks reveal themselves through queues: the step with growing backlog upstream of it is usually the bottleneck. Bottlenecks shift: fixing one moves the bottleneck somewhere else (usually visible within days). The Theory of Constraints prescribes a repeating cycle: identify, exploit, subordinate, elevate, repeat. The most common bottleneck-management mistakes are working on the wrong constraint (perceived but not actual) and breaking subordination (pushing work into the bottleneck faster than it can drain). Modern observability, process mining especially, has made bottleneck identification dramatically more rigorous than it was in the spreadsheet era.