Core Web Vitals are Google's set of measurable, user-focused page experience metrics. They replaced vaguer speed measures by defining specific moments in a page load that users actually notice. Three metrics form the set.
LCP — Largest Contentful Paint
What it measures: how long until the main visible content (usually a large image or heading) appears on screen.
Good: under 2.5 seconds. Needs improvement: 2.5–4 seconds. Poor: over 4 seconds.
What causes poor LCP:
- Large images that take a long time to download — see how to fix images slowing down your website.
- Slow server response (hosting quality).
- Render-blocking resources (scripts and stylesheets that load before the page can display anything).
How to improve: compress and resize the hero or banner image; it's the LCP element on most pages. Use a good host. Preload the LCP image if it's an <img> element.
INP — Interaction to Next Paint
What it measures: how quickly the page responds to user clicks, taps and keyboard input.
Good: under 200ms. Needs improvement: 200–500ms. Poor: over 500ms.
What causes poor INP:
- Heavy JavaScript that runs on the main thread and blocks responses.
- Too many third-party scripts (analytics, chat, ad tags).
- Large DOM trees (very complex page structures).
How to improve: reduce and defer JavaScript; audit third-party scripts for anything unnecessary.
CLS — Cumulative Layout Shift
What it measures: how much visible page elements jump around while the page loads. A button that moves as you're about to click it is a classic CLS problem.
Good: under 0.1. Needs improvement: 0.1–0.25. Poor: over 0.25.
What causes poor CLS:
- Images without defined dimensions in HTML — the browser doesn't know how much space to reserve until the image loads.
- Late-loading ads that push content down.
- Fonts that swap and cause text to reflow.
- Dynamic content inserted above existing content.
How to improve: set explicit width and height on every image; reserve space for ads; preload fonts.
Checking Your Scores
SEO Checker covers on-page basics. For Core Web Vitals specifically, Google Search Console (real user data from your visitors) and PageSpeed Insights (URL-by-URL lab test) are the standard tools. Field data (real users) and lab data (synthetic test) can differ — both are worth reviewing. See what is page speed and why does it matter for SEO for the broader context.
Why CLS Is Often Ignored Until It Hurts Visitors
Layout shift (CLS) gets less attention than LCP or INP because the developer's experience doesn't match the visitor's. Developers often test on fast connections where images and fonts load before the page is visible, so there's no shift. On slower connections, the page paints before images arrive, and content jumps when they load. Setting explicit width and height on all images — not just the hero, but thumbnails, icons and any img element — is the most consistent CLS fix available. For images specifically and their effect on both CLS and LCP, see how to fix images slowing down your website and how image size affects page speed.
