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Backend question · System design & architecture › Logging, metrics & tracing

An API has an average latency of 80 ms and a p99 latency of 2 s. Which statement is correct?

1% of requests take 2 s or more, and a page that makes many such calls will often hit one of them

p99 (the 99th percentile) means 99% of requests are faster than 2 s, and 1% are at least that slow. So "99% of requests take about 2 s" is wrong: it reverses the meaning.

Averages hide this slow tail. The tail matters most when one page makes many backend calls. With 100 calls, the chance that at least one is a slow p99 request is about 63%. So "Since the average is low, the 2 s requests are noise and can be ignored" is wrong: real users will often feel that delay.