Lead with measurement: "I install Query Monitor, check the queries panel for slow or duplicate queries, then check the hooks panel for expensive actions. Object caching with Redis usually gives the biggest single improvement."
Systematic approach: start with Query Monitor plugin to identify slow queries, excessive hooks, and HTTP API calls. Common issues: N+1 queries from WP_Query in loops, missing object caching (add Redis or Memcached), unoptimised images, too many plugins, no page caching, and render-blocking assets. Solutions: implement persistent object cache, add page-level caching (WP Super Cache, WP Rocket, or a reverse proxy), use transients for expensive operations, lazy-load images, optimise autoloaders, and reduce query counts. Strong candidates discuss: the difference between page caching and object caching, when to use transients versus the object cache, and database query optimisation with custom indexes.
Senior question. Developers who jump to installing a caching plugin without diagnosing the root cause are treating symptoms. Those who can read Query Monitor output and systematically address database, caching, and asset loading issues are production-ready.