What are percentiles (90, 95, 99) in measuring API response speed?

Percentiles (for example, the 90th, 95th, 99th) when measuring API response speed indicate the response time value below which a certain percentage of all measured responses fall.

For example, the 95th percentile means that 95% of all requests were processed faster than this time, and 5% slower.

This helps to understand not only the average response time but also the system's behavior in worst-case scenarios, identifying rare but significant delays.

Example: if the 99th percentile is 500 ms, it means 99% of requests respond faster than 500 ms, and 1% slower, which is important for assessing service quality and user experience.