Latency
The delay between asking a system for something and getting the answer, usually measured in milliseconds.
What it tells you about a developer
A developer who talks in milliseconds and percentiles has worked somewhere speed was a feature: trading, gaming, search, ads, or any consumer product at scale. Latency work is measurable, which is why it leaves a paper trail. A before-and-after with real figures is one of the cleanest proofs of senior performance engineering there is.
Latency is why some pages feel instant and others feel slow. Distance sets a hard floor on it: data moves at near light speed, so a request from Sydney to a server in Virginia takes around 200 milliseconds round trip before any code runs. Teams cut latency by placing servers near users, caching answers, and tuning slow code. Engineers quote it as percentiles, where p99 describes the slowest one percent of requests.