Core Web Vitals describe three different parts of perceived website quality: how fast the most important content appears, how quickly an interaction is answered, and how stable the layout stays.
LCP: When is the main content visible?
Largest Contentful Paint measures the point at which the largest visible content element was rendered. Often that's a large image or a heading. A slow LCP can be caused by large files, late server responses, blocking styles, or a main element that is only generated by JavaScript.
INP: How fast does the interface respond?
Interaction to Next Paint looks at interactions throughout the entire visit. Long JavaScript tasks, heavy event handlers, and unnecessary recalculations can delay the response. The analysis therefore doesn't start with animations but with the longest tasks in the browser profile.
CLS: Does the page stay in place?
Cumulative Layout Shift rates unexpected shifts. Common causes are images without fixed dimensions, content loaded late above the visible area, or fonts whose swap strongly changes text width.
Separate field and lab data
Lighthouse produces lab data under defined conditions. Chrome User Experience Report data reflects real visits. Both are useful but answer different questions. A single Lighthouse run is therefore not reliable proof of the real experience of all users.
A sensible order of checks
- Identify real field data and affected page types.
- Determine the concrete LCP element and its dependencies.
- Measure long tasks around real user interactions.
- Locate layout shifts in the performance profile.
- Re-check changes under comparable conditions.