Bottleneck
Etymology
From the literal narrow neck of a bottle, which limits how fast liquid can pour no matter how full the bottle is. The image was borrowed for any choke point in a system, first in traffic and manufacturing. It endures because the physical picture is so exact.
noun · /BOT-l-nek/
The single point in a process where flow gets constricted, limiting the speed of everything else.
What it names
The one constraint worth fixing, cutting through the temptation to optimize parts that aren't actually holding you back.
Example
Approvals were the bottleneck — engineers finished fast but waited days for a sign-off.
Related idea
A bottleneck limits throughput the way the slowest hiker sets a group’s pace. Speeding up anything but the bottleneck just piles up work in front of it — a common and expensive mistake.