Core Web Vitals describe three different parts of perceived website quality: how quickly the most important content appears, how promptly an interaction gets a response, and how stable the layout stays.
LCP: when is the main content visible?
Largest Contentful Paint measures the point in time when the largest visible content element has rendered. Often that's a large image or a headline. A slow LCP can result from large files, late server responses, render-blocking styles, or a main element that's only produced by JavaScript.
INP: how fast does the interface respond?
Interaction to Next Paint looks at interactions across the entire visit. Long JavaScript tasks, expensive 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 evaluates unexpected movement. Common causes are images without fixed dimensions, content that loads later above the visible area, or fonts whose swap significantly changes text width.
Separating field and lab data
Lighthouse produces lab data under defined conditions. Chrome User Experience Report data reflects real visits. Both are useful, but they answer different questions. A single Lighthouse run is therefore not solid proof of the real experience for all users.
A sensible order of checks
- Identify real field data and the page types affected.
- Determine the specific 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.