Speed is not a technical vanity metric: it is revenue. A slow page loses the visitor before it even gets a chance to convince. This guide explains the three Core Web Vitals metrics, what each one measures, what the targets are, and what to do, in practice, to hit them.
The three metrics and their targets
| Metric | What it measures | Target (good) |
|---|---|---|
| LCP | time until the largest visible element appears | under 2.5 s |
| INP | the page's response to user interactions | under 200 ms |
| CLS | visual stability of the layout | below 0.1 |
These targets apply to the 75th percentile of real visits, measured in the field. Lab testing is useful for diagnosis; field data is what actually counts.
How to improve LCP
- Identify the LCP element, almost always the hero image or the main headline.
- Serve images in WebP or AVIF, with
widthandheightdeclared. - Remove
loading="lazy"from the top element and usefetchpriority="high". - Preload the critical image and the fonts used above the fold.
- Apply
font-display: swapso text rendering isn't blocked.
How to improve INP
- Reduce third-party JavaScript: every tag you add charges a toll on interaction.
- Break up long tasks and defer anything not needed on the first screen.
- Load chat, map, and video scripts only on interaction.
- Avoid animations that trigger layout on every frame.
How to get CLS to zero
Reserve space for everything that loads later: images, banners, cookie notices, and ads. Fonts with metrics very different from their fallback also shift the text, use size-adjust or a closer fallback.
How to measure with confidence
Field
Chrome's user experience report and Search Console show the real behavior of your visitors.
Lab
Lighthouse and WebPageTest help isolate the cause. Combine them with GA4 events to connect speed and conversion by page.
On commercial pages, handling performance is part of the same cycle described in CRO and it directly affects landing page structure.
Core Web Vitals checklist
- LCP element identified for each page template.
- Images in WebP/AVIF with declared dimensions.
- Hero image with high fetchpriority and no lazy loading.
- Fonts preloaded with font-display: swap.
- Third-party scripts audited and deferred.
- Space reserved for asynchronous elements.
- Field data monitored in Search Console.
Conclusion
Core Web Vitals is not a developer's checklist: it is conversion infrastructure. Fixing LCP, INP, and CLS increases the return on everything you're already running, without adding a real to media spend.
Frequently asked questions about core web vitals
What are Core Web Vitals?
They are three user experience metrics defined by Google: LCP, which measures the time until the largest visible element appears; INP, which measures responsiveness to interactions; and CLS, which measures the layout's visual stability.
What are the targets for LCP, INP, and CLS?
LCP under 2.5 seconds, INP under 200 milliseconds, and CLS below 0.1, always looking at the 75th percentile of real visits measured in the field.
Do Core Web Vitals influence Google ranking?
Yes, as part of the page experience signals. The direct impact on ranking is moderate, but the effect on conversion and abandonment tends to be much bigger.
How do I improve the LCP of a landing page?
Serve the main image in WebP or AVIF with declared dimensions, remove lazy loading from the top element, use a high fetchpriority, preload the critical image, and apply font-display swap.
What is the difference between lab data and field data?
Lab data comes from simulated tests, like Lighthouse, and is useful for diagnosis. Field data comes from real users and is what Google considers in its evaluation.
About the author
Yara Iencius
Partner and Head of Design & UX, KCLOCK
Conversion designer. Responsible for UX/UI, interfaces, landing pages, and e-commerce sites that turn strategy into a high-converting experience.
- #performance
- #technical seo
- #cro
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