Lead with measurement: "I profiled with Blackfire, found 60% of request time was N+1 queries, added eager loading, and reduced response time from 800ms to 120ms."
Strong answers mention profiling tools (Xdebug, Blackfire, Tideways) before optimising. Common optimisations: OPcache tuning, database query optimisation (N+1 queries, missing indexes), caching layers (Redis/Memcached), reducing autoloader overhead, preloading (PHP 7.4+), and JIT (PHP 8.0+). Best candidates describe a specific scenario with measurable before/after improvements.
Senior-level question. Developers who optimise without profiling are guessing. Those who can show a systematic approach to performance work are valuable. Ask what tools they use and whether they have production-level profiling experience.