What are web performance metrics and tools?

Key metrics and tools for measuring web performance: Metrics: LCP, INP, CLS (Core Web Vitals), FCP, TTFB, TTI (Time to Interactive), TBT (Total Blocking Time), Speed Index. Lab tools (synthetic, simulated): Lighthouse: built into Chrome DevTools; run in CI; audits performance, accessibility, SEO. WebPageTest: filmstrip, waterfall chart, advanced network simulation. Chrome DevTools Performance panel: flame chart, main thread analysis, layout shifts. Chrome DevTools Network panel: waterfall, timing, cache headers. Field data (real users): Chrome User Experience Report (CrUX): aggregated real-user data from Chrome users. Google Search Console: Core Web Vitals report from CrUX for your site. PageSpeed Insights: combines Lighthouse + CrUX data. Real User Monitoring (RUM): custom monitoring with web-vitals library: import { onLCP, onINP, onCLS } from "web-vitals"; onLCP(console.log). Lab data shows potential; field data shows real user experience. Always prioritize fixing field data (CrUX) since it is what Google uses for search ranking.