Always measure before optimising. Name specific tools you use to profile (browser devtools, APM, database EXPLAIN) and show you fix the biggest bottleneck first.
Strong answers follow a systematic approach: measure first (profiling, APM tools, browser devtools), identify the bottleneck layer (frontend rendering, network, backend processing, database queries), then optimise the biggest bottleneck. Specific techniques: query optimisation with EXPLAIN, N+1 query elimination, lazy loading, code splitting, connection pooling, and knowing when to cache versus when to fix the underlying issue.
Separates senior from mid-level developers. Juniors guess; seniors measure. Ask for a specific example where they improved performance and by how much. Quantified results show real experience.