Web performance is the discipline of making web pages load fast and stay responsive. The Core Web Vitals (LCP, INP, CLS) are now ranking factors for Google, which made performance a measurable concern at every product team rather than a nice-to-have.
The toolchain is mature — Chrome DevTools Performance panel, WebPageTest for synthetic testing, Real User Monitoring (RUM) via SpeedCurve / Calibre / DebugBear / open-source alternatives, and Lighthouse CI for regression detection. The actual work splits into easily-fixed surface issues (image sizing, font loading, render-blocking JS) and the harder ones underneath (JavaScript bundle splitting, third-party script audit, server response time).