Web performance encompasses the techniques and technologies that make websites load faster and respond more quickly to user interactions. At its core, it's measured through Core Web Vitals—Google's standardized metrics that quantify real user experience—and optimized through strategies like caching, compression, lazy loading, and resource prioritization. Modern web performance isn't just about initial page load anymore; it's about sustained responsiveness throughout the entire session, measured by metrics like Interaction to Next Paint (INP) that track how quickly a page responds to every click, tap, and keystroke. The key mental model: every millisecond of delay compounds across millions of users, making performance optimization one of the highest-ROI investments in web development.