How to Measure and Improve Core Web Vitals

A website that loads is not necessarily a website that feels fast. Users care about when the main content appears, how quickly the interface responds, and whether the layout moves unexpectedly. Core Web Vitals quantify these parts of the experience.

Three metrics to understand

Google evaluates these thresholds at the 75th percentile, separately for mobile and desktop.

Step 1: Measure with PageSpeed Insights

1. Enter the complete page URL.
2. Review mobile and desktop separately.
3. Start with real-user field data when it is available.
4. Use Lighthouse lab data to diagnose likely technical causes.

Field data explains what real visitors experience. Lab data helps explain where a problem may be introduced.

Step 2: Test representative pages

Do not test only the homepage. Include article, category, product, landing, and form pages. Record the URL, device profile, deployment version, and result so later measurements are comparable.

Step 3: Improve LCP

• Compress images and serve responsive WebP or AVIF where appropriate.
• Do not lazy-load the main above-the-fold image.
• Reduce server response time with caching and query optimization.
• Remove render-blocking CSS and JavaScript.
• Preload only genuinely critical resources.

Step 4: Improve INP

• Break long JavaScript tasks into smaller work units.
• Remove unused code and delay nonessential third-party scripts.
• Keep event handlers small and avoid unnecessary rendering.
• Show immediate visual feedback before completing expensive work.

Step 5: Reduce CLS

• Declare dimensions or aspect ratios for images, videos, and iframes.
• Reserve space for banners, advertising, and asynchronous widgets.
• Avoid inserting content above what the user is currently reading.
• Use a suitable fallback font to reduce text reflow.

Step 6: Prioritize

Start with high-traffic templates and conversion paths. Fix high-impact, low-risk issues first: oversized images, missing dimensions, unnecessary third-party scripts, and ineffective caching.

Step 7: Measure again

Lab results can change immediately, while field data needs time to accumulate. Monitor Search Console and PageSpeed Insights after deployment, and connect technical improvements to bounce rate, engagement, and conversion.

Quick checklist

• Tested mobile and desktop.
• Separated field data from lab data.
• Optimized the main image.
• Reserved media and widget space.
• Reviewed third-party JavaScript.
• Recorded before-and-after results.

References

• Google Search Central: Core Web Vitals
• web.dev: Web Vitals