Bottleneck

The single slowest component in a system that caps overall speed, no matter how fast every other part runs.

Mental model: Like one narrow lane slowing an entire highway.

Example: A checkout flow that's fast everywhere except one synchronous call to a slow tax API, which caps the whole flow's speed.

In practice: Profiling tools exist specifically to find the real bottleneck, since intuition about what's actually slow is frequently wrong.

• often confused with Hot path