They answer different questions
Performance testing measures how a system behaves under expected or planned load. It answers questions about latency, throughput, error rate, and user experience while the system is supposed to remain healthy.
Stress testing pushes beyond normal expectations to learn where the system degrades, what fails first, and whether recovery is clean. Both are useful, but they should not be treated as the same exercise.
When to use performance testing
Use performance testing when you need to tune an API, application flow, database path, cache policy, or release candidate. The goal is stable operation under realistic load. You should measure latency percentiles, errors, queue depth, CPU, memory, database behavior, and user outcomes.
Performance testing is often the safest starting point because it uses controlled request patterns and makes application bottlenecks visible.
When to use stress testing
Use stress testing when you need to validate limits and resilience. That may include DDoS mitigation, packet handling, game-server pressure, WAF behavior, connection-state exhaustion, or provider scrubbing. The goal is not to keep graphs pretty. The goal is to find the boundary safely.
Stress testing needs stricter scope and stop conditions because it intentionally approaches failure.
How they work together
A mature program starts with performance baselines, then runs bounded stress tests, then fixes the weakest layer and repeats. The performance baseline explains normal behavior. The stress test explains what breaks under pressure.