Start with clear requirements: "page load under 2 seconds at 1000 concurrent users." Without specific targets, performance testing is just generating charts.
Strong answers cover: defining performance requirements (response time, throughput, concurrency), tool selection (JMeter, k6, Gatling, Locust), test environment considerations, realistic load profiles, baseline establishment, bottleneck identification, and reporting. Best candidates discuss continuous performance testing in CI, not just pre-release.
Advanced QA skill. Not all QA roles require deep performance testing, but understanding the approach is important. Ask about the most interesting performance bottleneck they have found.