The bottleneck is the process in a chain with the lowest capacity, which thereby limits the throughput of the entire system. An hour lost at the bottleneck is an hour lost for the whole factory — capacity gained everywhere else is an illusion.
The Theory of Constraints (Goldratt) gives the recipe: (1) find the bottleneck, (2) exploit it fully — no stops, no waiting, no scrap, (3) subordinate the rest of the system to its rhythm, (4) elevate its capacity, (5) find the new bottleneck and repeat.
Bottlenecks reveal themselves by accumulation in front and idle time behind. But beware of wandering bottlenecks: unstable processes move the constraint around from day to day — stabilisation comes before optimisation.
To find the constraint without a full mapping, walk backwards through the chain and find where work piles up the most — the process just after the biggest pile is usually the bottleneck. And remember it is rarely a machine: in many companies it sits in an approval, a decision or one person everyone is waiting for.