A road at 80% capacity isn't 80% as smooth. It's where traffic jams begin.
Queueing theory gives the shape. As utilisation climbs, waiting time doesn't rise steadily — it curves upward and then goes vertical.
| Utilisation | Average wait |
|---|---|
| 50% busy | ≈ 1× service time |
| 80% busy | ≈ 4× |
| 90% busy | ≈ 9× |
| 95% busy | ≈ 19× |
Nothing broke between 80 and 95. The servers are healthy. The code didn't change. Only the gaps disappeared.
Arrivals aren't evenly spaced. Idle capacity isn't waste — it's the room a burst lands in.