Core Web Vitals Explained: What They Are, How to Measure Them, and Why They Matter for React Apps

If you've seen the term "Core Web Vitals" and kept scrolling, this article is for you. It's not just SEO jargon. These three metrics are the clearest signal we have for whether a web app feels fast to a real user — and they're measurable directly from your React code. This article covers what the three metrics actually mean, how to measure them without any external tools, and what to do when they're bad. What Are Core Web Vitals? Core Web Vitals are three metrics defined by Google to measure user experience from a loading and interactivity perspective. They're based on real user data, not synthetic benchmarks. The three metrics: Metric Measures Good threshold LCP — Largest Contentful Paint Loading speed ≤ 2.5s FCP — First Contentful Paint Time to first visible content ≤ 1.8s CLS — Cumulative Layout Shift Visual stability ≤ 0.1 There's a fourth metric worth knowing: INP (Interaction to Next Paint) , which replaced FID (First Input Delay) in 2024. INP measures how responsive the page fee