What does Web Vitals do?

Web Vitals is a set of key metrics from Google that measure the quality of user experience on a site. They show how fast, stable, and responsive the page is from a real user's point of view.

---

#Main Web Vitals metrics:

1. LCP (Largest Contentful Paint)

• when the largest visible element loads
• reflects: how fast the main content renders
• norm: < 2.5 sec

2. FID (First Input Delay)

• the delay between the user's first action and the page's response
• reflects: interface responsiveness
• norm: < 100 ms

3. CLS (Cumulative Layout Shift)

• the sum of all unexpected element shifts during loading
• reflects: visual stability
• norm: < 0.1

---

#Additional (extended) metrics:

• FCP (First Contentful Paint): when the first content appears
• TTFB (Time To First Byte): when the first byte of the server response arrives
• INP (Interaction to Next Paint): a replacement for FID with more precise interaction measurement

---

#What Web Vitals does in practice:

• Measures key UX metrics on real devices (via the browser)
• Sends them to analytics (for example, to Google Analytics, BigQuery, and so on)
• Helps identify where the site actually slows down, not just in synthetic tests
• Serves as the basis for scoring in Lighthouse, PageSpeed Insights, and the Chrome UX Report (CrUX)

---

Conclusion: Web Vitals is the minimal required set of metrics by which Google (and the user) judges how fast, responsive, and non-frustrating a site is.