# Web performance and Core Web Vitals: where to start

> Core Web Vitals are three metrics Google uses to measure real user experience: loading speed, interaction responsiveness and visual stability. This guide summarises what the metrics mean and where to start improving.

**In short:**

- Aim for LCP ≤ 2.5 s, INP ≤ 200 ms, CLS ≤ 0.1.
- Confirm measurements with real user (field) data.
- Image size and the amount of JavaScript are the most common causes.
- Content in the first HTML helps both speed and visibility.

## What do the three metrics measure?

- LCP (Largest Contentful Paint): how long it takes for the page's main content to appear. Good: 2.5 seconds or less.
- INP (Interaction to Next Paint): how quickly the page responds to user interactions. Good: 200 milliseconds or less.
- CLS (Cumulative Layout Shift): unexpected shifting of elements while the page loads. Good: 0.1 or less.

Evaluation uses the 75th percentile of real users, so a test on a single fast computer is not enough.

## Measure first

Lab tools (such as Lighthouse) help find problems; real user data (CrUX, the field data in PageSpeed Insights) is what Google sees. Follow both and move forward by measuring the effect of each change.

## Improving LCP

- Serve the main image at the right size and in a modern format (e.g. WebP/AVIF).
- Reduce or defer large JavaScript and CSS files that block the main content.
- Shorten server response time; serve the page from cache where possible.
- Make the main content arrive in the first HTML; content built later in the browser is delayed.

## Improving INP

- Break up long-running JavaScript tasks.
- Avoid unnecessary re-rendering on interaction.
- Limit third-party scripts; every extra script can affect responsiveness.

## Improving CLS

- Set width and height on images and videos.
- Reserve space for ads and embedded content.
- Choose the fallback font and display strategy carefully so content does not shift while fonts load.

## Set a performance budget

Performance is not a job done once. Setting an upper limit on total file size and script count per page stops speed from silently degrading as new features are added.

## Conclusion

VeriSet A.Ş. builds modern web applications together with performance and accessibility goals. For more, see our web platforms area page.

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https://veriset.org/en/guides/web-performance-and-core-web-vitals/ · destek@veriset.org