Always start by profiling to confirm the bottleneck. Then discuss your invalidation strategy — it is the hardest part and interviewers want to hear you address it directly.
Strong answers discuss: identifying the bottleneck first (do not cache prematurely), cache layers (browser, CDN, application, database query cache), cache invalidation strategies (TTL, event-driven, write-through), and the risks — stale data, cache stampedes, memory pressure, and the complexity of invalidation. Best candidates quote the "two hard things" and explain their practical approach to invalidation.
Tests systems thinking. Caching is deceptively simple to add and notoriously hard to get right. Candidates who only discuss adding a Redis layer without addressing invalidation lack depth. Ask: "How do you handle cache warming after a deploy?"