Core Web Vitals: assess website speed and usability

Understand the three measures

Largest Contentful Paint (LCP) measures how quickly the largest visible content element is rendered. Interaction to Next Paint (INP) measures responsiveness to interactions. Cumulative Layout Shift (CLS) measures unexpected visual movement.

Google’s good thresholds are LCP at or below 2.5 seconds, INP at or below 200 milliseconds and CLS at or below 0.1, assessed at the 75th percentile of real page views. Review mobile and desktop separately.

Choose representative pages

Include the homepage, a service page, a guide or article and the contact page. Add any template with a different layout or integration. Record the device, connection, page version and test conditions.

Start with the pages that matter to customers. A fast empty page tells you little about a service page containing images, forms and third-party scripts. The technical SEO review helps connect these checks with the rest of the site.

Separate field observations from laboratory tests

Field data reflects real visitors over the reporting period. A lab test runs under controlled conditions and helps investigate a specific load. Use each for its purpose. New or low-traffic pages may not have enough field data.

After a change, repeat the same lab conditions and inspect the actual element or task involved. Review the available field data later as the reporting period includes the changed version. Record dates so that the comparison is understandable.

Investigate the largest loading element

Identify the LCP element first. If it is a hero image, check its dimensions, format, responsive sizes and request priority. The primary image should be available early. Images farther down the page can usually load later.

Also review server response, render-blocking styles and font loading. Each change should address an observed cause. Compare the image quality on the devices people use and keep explicit dimensions or an aspect ratio.

Test interactions and visual movement

Try opening navigation, expanding answers and completing the form on a mobile device. Look for long tasks and scripts that keep the browser busy. Review third-party scripts and the pages on which they are loaded.

Reserve space for images, embeds and dynamically inserted elements. Check how fonts change the layout. Repeat the interaction with the consent banner visible and after the visitor makes a choice.

Verify the customer journey

A performance change can affect navigation, forms, consent and measurement. Test those functions after changing script loading, caching or asset optimisation. Include visitors who are logged out and both language versions.

Record what changed, the measured effect and any regression. WordPress SEO covers the interaction between themes, plugins and hosting. Conversion tracking helps check that the enquiry path still works.

Sources

• Google web.dev: Web Vitals
• Google web.dev: Core Web Vitals thresholds

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