Performance Testing Interview Questions

Load testing checks expected and peak traffic against agreed targets. Stress testing goes beyond capacity to find the breaking point and how the system fails and recovers. Soak testing holds realistic load for hours to expose leaks. Spike testing applies sudden surges to check scaling and recovery.

Latency distributions are skewed with a long tail, so the average hides slow requests and is pulled around by outliers. p95 means 95% of requests were at or below that value. Percentiles describe what real users experience, and tail latency (p99) matters because one page triggers many requests.

Derive it from production data: peak arrival rate, the mix of user journeys and their proportions, think time, data variety and session behaviour. Use an open model (fixed arrival rate) for public traffic and a closed model (fixed concurrent users) for bounded user groups, and agree targets up front.

Define scenarios with an executor such as ramping-arrival-rate, use checks to validate responses without stopping the test, and set thresholds on metrics like p(95) of http_req_duration and http_req_failed so the run exits non-zero in CI when targets are missed. Tag requests to get per-endpoint results.

Confirm the load generator is not the limit, then correlate the latency curve with server telemetry using the USE method: CPU, memory and GC, thread and connection pools, database queries and locks, downstream calls. Use APM traces to see where time goes, change one thing, and re-test.