Core Web Vitals measure the experience your visitors feel: how fast content loads, how quickly it responds, and how stable it stays. The three current metrics — Largest Contentful Paint (LCP), Interaction to Next Paint (INP), which replaced First Input Delay in 2024, and Cumulative Layout Shift (CLS) — each capture a distinct dimension of page experience that directly affects user satisfaction and, increasingly, search ranking signals.
Largest Contentful Paint measures the time until the largest visible content element — typically a hero image or heading — is fully rendered. A good LCP score is under 2.5 seconds measured at the 75th percentile of field data. The most common LCP optimization opportunities are server response time reduction, eliminating render-blocking resources, optimizing the LCP image with proper sizing, preloading, and modern formats, and reducing third-party script impact on initial page load.
Interaction to Next Paint replaced First Input Delay as the interactivity metric and measures the time between any user interaction and the next visual response. INP captures slow interactions throughout the page lifecycle rather than just the first interaction, making it a more comprehensive measure of JavaScript execution efficiency. Common INP problems include long tasks that block the main thread, excessive event handler execution, and third-party widgets that delay interaction response.
Cumulative Layout Shift measures unexpected layout movement that disrupts reading or causes accidental clicks. A good CLS score is under 0.1. The most common causes are images and embeds without explicit width and height attributes, dynamically injected content above existing content, and web fonts that cause text reflow when they load. Setting explicit dimensions on all media elements and reserving space for ad slots and embeds eliminates the majority of CLS issues.
Optimise your largest content element, defer non-critical scripts, and reserve space for images to eliminate layout shift. Start with the LCP element on your most important page templates — typically the hero image on the homepage and article pages. Preload the LCP image with a high-priority fetch directive, serve it from a CDN geographically close to your users, and use WebP or AVIF format at appropriately sized dimensions.
Measure with field data, not just lab tests. Real-user metrics reveal the performance your audience actually experiences. PageSpeed Insights shows both lab data (Lighthouse simulation) and field data from the Chrome User Experience Report for the same URL. The field data is what Google uses for ranking signals, and it often tells a different story than the lab test — particularly for sites with global audiences experiencing varying network conditions.
Third-party scripts are among the most significant and hardest-to-control sources of Core Web Vitals degradation. Analytics platforms, chat widgets, advertising tags, and social media embeds can individually add hundreds of milliseconds to LCP and cause significant INP delays. Audit every third-party script running on your site, remove any that are not actively delivering value, and load remaining ones with defer or async attributes to minimize their main thread impact.
