Core Web Vitals: How to Measure and Improve Site Speed
Short answer
Core Web Vitals are Google's three user-experience metrics. LCP (Largest Contentful Paint) should be under 2.5 seconds, INP (Interaction to Next Paint) under 200 milliseconds, and CLS (Cumulative Layout Shift) under 0.1, measured at 75% of real visits. They are one ranking signal, but they carry less weight than content relevance.
What are the three Core Web Vitals?
| Metric | What it measures | Good | Needs improvement | Poor |
|---|---|---|---|---|
| LCP Largest Contentful Paint | How fast the main content appears | ≤ 2.5 s | ≤ 4 s | > 4 s |
| INP Interaction to Next Paint | How fast the page responds after a click | ≤ 200 ms | ≤ 500 ms | > 500 ms |
| CLS Cumulative Layout Shift | Whether the layout jumps around | ≤ 0.1 | ≤ 0.25 | > 0.25 |
The benchmark is the 75th percentile: a metric passes only if 75% of real visits reach “Good.” The thresholds come from web.dev’s Web Vitals guide.
What’s the difference between lab data and field data?
- Lab data (Lighthouse): a single test on a simulated device and network. Useful for debugging during development, but not representative of real users.
- Field data (CrUX, the Chrome User Experience Report): statistics from real Chrome users. This is what Google uses for ranking.
A new site with too little traffic has no field data, so you can only look at lab data at first.
How do you improve each one?
LCP is too slow
- Reduce server response time (TTFB under 800 milliseconds is recommended) and use a CDN.
- Don’t lazy-load the main image (don’t add
loading="lazy"to above-the-fold images), and use modern formats such as WebP and AVIF. - Reduce render-blocking CSS and JavaScript.
INP is too slow
- Reduce long JavaScript tasks on the main thread.
- Remove third-party scripts you don’t need (ads, tracking codes, and chat widgets are usually the culprits).
- Break heavy computation into smaller chunks, or defer it until after user interaction.
CLS is too high
- Set
widthandheighton images and videos so the browser can reserve space. - Reserve fixed-size containers for ads and embeds.
- Use
font-display: optionalor preload web fonts to avoid layout shifts when fonts swap.
Which tools should you use to measure?
- PageSpeed Insights: shows field data (if available) and lab data side by side.
- Search Console’s “Core Web Vitals” report: shows which URLs across the site fail.
- The Performance panel in Chrome DevTools: pinpoints which code is causing delays.
Why are static sites especially fast?
One of the reasons this site chose Astro: by default it builds pages as plain HTML and ships no JavaScript unless you explicitly need it. With no JavaScript to download and execute, LCP and INP are good by default. This also helps GEO, because AI crawlers can read the content without executing JavaScript (see GEO: Getting ChatGPT and Perplexity to Cite Your Content).
FAQ
Does a PageSpeed Insights score need to be 100 to count as good?
No. The Lighthouse score is a simulated lab value, while Google's rankings use Core Web Vitals data from real users. Having all three metrics in the "Good" range is enough.
Is FID still a Core Web Vital?
No. FID was replaced by INP in March 2024. INP measures the responsiveness of all interactions throughout a visit, making it more complete than FID.
How much does site speed affect rankings?
Google says page experience is one ranking signal, but content relevance matters far more than speed. When several pages have similar content quality, the one with the better experience may win.
→ Post-Launch Measurement: Search Console, Bing & AI Citations