API performance testing is crucial to ensure application programming interfaces (APIs) can handle expected and unexpected loads without compromising the user experience. It evaluates an API's speed, responsiveness, reliability, and scalability under various conditions, simulating real-world events. Unlike functional testing, which verifies what an API does, performance testing addresses how it performs. Key metrics that matter include response time, error rate, latency, and CPU and memory usage. Interpreting test results requires establishing a baseline, evaluating response time trends, understanding error rate patterns, and correlating metrics for deeper insights. Best practices for testing at scale include getting started early and continuously, simulating real-world scenarios, using the right tools for the job, testing all critical endpoints, avoiding common pitfalls, and iterating to improve performance. By following these guidelines, developers can ensure their APIs are robust, dependable, and provide a great user experience even under pressure.