Performance testing is used to determine the speed and effectiveness of software. It involves testing a system under pressure to analyze response times, throughput, and identify bottlenecks. The key aspects of performance testing include load testing by simulating peak usage, stress testing beyond normal limits, tuning to optimize performance, and benchmarking to compare a product's performance against competitors. The performance testing process involves planning test objectives, recording usage scenarios, modifying scripts to simulate realistic loads, executing tests while monitoring performance, and analyzing results to identify issues.