Embedding and web termsUpdated
Core Web Vitals
Also called: CWV, LCP, INP, page experience metrics
Definition
Core Web Vitals are three metrics that describe a page's real-world user experience: Largest Contentful Paint for loading, Interaction to Next Paint for responsiveness and Cumulative Layout Shift for visual stability. Each has a published “good” threshold.
The three metrics
INP became a stable Core Web Vital in 2024, taking the place of First Input Delay. web.dev says stable metrics change at most once a year.
| Metric | Measures | Good |
|---|---|---|
| Largest Contentful Paint (LCP) | How soon the main content appears | 2.5 seconds or less |
| Interaction to Next Paint (INP) | How quickly the page responds to taps, clicks and key presses | 200 milliseconds or less |
| Cumulative Layout Shift (CLS) | How much content moves unexpectedly | 0.1 or less |
Field data vs lab data
The thresholds apply to the 75th percentile of real page loads, split by mobile and desktop. That real-user data comes from the Chrome User Experience Report and shows up in PageSpeed Insights and Search Console's Core Web Vitals report. Lab tools such as Lighthouse and Chrome DevTools are better for debugging, but Lighthouse loads pages without a user, so it can't measure INP and reports Total Blocking Time instead.
A new or low-traffic page may not have enough real visits for field data yet. Use lab tools first, then confirm with field data once it appears.
Where third-party embeds fit
- LCP: an embed rarely is the largest element, unless it sits at the top of the page. Its scripts can still compete with your own content for bandwidth.
- INP: heavy scripts running on your page can delay responses to clicks. A loader that only creates an iframe and leaves the rendering to it adds little script to your own page.
- CLS: embeds that change size after loading are a common cause; see Cumulative Layout Shift.
To check a widget's effect on your own site, the page speed question describes a simple before-and-after test.
Related terms
- Cumulative Layout Shift (CLS)
- Cumulative Layout Shift (CLS) is a Core Web Vitals metric for visual stability: it measures how much visible content moves unexpectedly while a page is open. web.dev rates a score of 0.1 or less as good and anything above 0.25 as poor.
- Lazy loading
- Lazy loading means the browser waits to load an image or iframe until the visitor scrolls near it, instead of fetching everything as the page opens. In HTML you turn it on by adding loading="lazy" to <img> and <iframe> elements.
- Script tag (async and defer)
- A script tag, <script>, adds JavaScript to a web page. By default an external script pauses HTML parsing while it downloads and runs; the async and defer attributes let the browser keep parsing and run the script later.
- Structured data (JSON-LD)
- Structured data is machine-readable markup that describes what a page is about, such as an article, product, event or organization, using a shared vocabulary like schema.org. Google recommends the JSON-LD format and uses structured data to understand pages and show rich results.
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