Performance testing is the process of determining the speed, responsiveness, and stabilityof a system under a given workload1. Performance testing should be started as early as possible in the software development lifecycle, preferably during unit and integration testing, to identify and resolve performance issues before they become costly or risky2. Starting performance testing early can also help to validate the performance requirements, design, and architecture of the system, as well as to optimize the performance testing strategy and scope3. Therefore, option A is the correct answer. Option B is incorrect because deferring the start of performance testing until all functional defects have been resolved can delay the detection and resolution of performance issues, increase the cost and effort of performance testing, and reduce the confidence and quality of the system4. Option C is incorrect because leveraging end users to do unit-level performance testing and automated tools for system-level performance testing can introduce inconsistency, bias, and inefficiency in the performance testing process, as well as compromise the reliability and validity of the performance test results. Option D is incorrect because requiring all performance tests to pass before starting functional testing can create unrealistic or unnecessary expectations, as well as hinder the progress and feedback of the functional testing activities. References: 1: ISTQB Glossary, Performance Testing 2: ISTQB Certified Tester - Performance Testing (CT-PT)3 3: ISTQB Performance Testing - TesterYou4 4: Performance Testing - ISTQB not-for-profit association : ISTQB - PERFORMANCE TESTING : Performance Testing - ISTQB not-for-profit association