Core Web Vitals

Core Web Vitals are three metrics Google publishes to describe how a page feels to the people using it. Largest Contentful Paint measures how long the main content takes to appear. Interaction to Next Paint measures how quickly the page responds when somebody clicks or types, and it replaced First Input Delay in 2024. Cumulative Layout Shift measures how much the layout moves after it has been drawn. The published thresholds for a good rating are 2.5 seconds, 200 milliseconds and 0.1 respectively, assessed at the seventy-fifth percentile of real visits, so the verdict describes the slower quarter of your traffic rather than a typical one.

Two consequences follow commercially. The first is that these are field measurements taken from real devices on real connections, so a laboratory run on a developer laptop can look excellent while the reported figures fail. The second is that they are a modest input to search ranking and a much stronger input to whether anybody stays: a page that shifts under a thumb mid-tap loses the tap, and a slow first render loses the visit before any of your content gets a chance to argue for itself.

Where each of the three usually goes wrong

• Loading is normally a render-blocking resource, an unoptimised hero image, or a web font that delays the very text it was chosen to display.
• Responsiveness is normally long JavaScript tasks occupying the main thread, often hydration work or a third-party tag running at the exact moment somebody first interacts.
• Stability is normally images and embeds without reserved dimensions, or a consent banner and a late font arriving after first paint and pushing the page down under the reader.

Improving a synthetic score is not the same as improving the experience, because the figures that count are collected from your visitors’ hardware, which is slower and more varied than anything on your desk.