Caching is one of the highest-leverage performance improvements available to an API, and one of the least consistently implemented. A response that is cached at the right layer — in a CDN, a reverse proxy, or the client itself — eliminates a server round trip entirely. At scale, that elimination compounds: fewer database queries, lower infrastructure cost, faster responses for every consumer. The HTTP specification provides a complete, standardized caching system. Most APIs use it only partially, leaving significant headroom unrealized.