In 2021, Google introduced Core Web Vitals as an official ranking factor. Since then, speed and user experience are no longer just best practices — they are metrics with a direct impact on rankings. This guide explains what each Core Web Vital measures, what thresholds separate a good experience from a bad one, and why Google gives them such importance.
What are Core Web Vitals?
Core Web Vitals are a subset of Google's Web Vitals: three specific metrics that measure fundamental aspects of the user experience on the web — loading speed, interactivity, and visual stability. Google chose them because they have a demonstrated statistical correlation with page abandonment and user satisfaction.
Core Web Vitals are measured primarily with real field data (Chrome User Experience Report / CrUX), not just lab tests. This means what matters is the real experience of your actual users, not just how the page performs under ideal conditions.
LCP: Largest Contentful Paint (Loading)
LCP measures the time it takes for the largest visible content element in the initial viewport to render. This is usually a hero image, a large text block, or a video. It is the metric that best represents the user's perception of loading speed.
| LCP threshold | Rating | User experience |
|---|---|---|
| ≤ 2.5 seconds | ✅ Good | User perceives the page as fast |
| 2.5 – 4.0 seconds | ⚠️ Needs improvement | Noticeable load, some users abandon |
| > 4.0 seconds | ❌ Poor | High abandonment rate, ranking impact |
The most common causes of high LCP are: unoptimized or non-preloaded images, slow servers (high TTFB), CSS or JavaScript blocking rendering, and web fonts without font-display: swap.
INP: Interaction to Next Paint (Interactivity)
INP (Interaction to Next Paint) replaced FID (First Input Delay) as the official metric in March 2024. While FID only measured the delay before the first click, INP measures the latency of all user interactions with the page throughout the entire visit — clicks, keyboard presses, and taps. It is a much more representative measure of real interactivity.
| INP threshold | Rating | User experience |
|---|---|---|
| ≤ 200 ms | ✅ Good | Response feels instantaneous |
| 200 – 500 ms | ⚠️ Needs improvement | Perceptible lag, degraded experience |
| > 500 ms | ❌ Poor | Page feels blocked or broken |
The main causes of high INP: JavaScript with long tasks on the main thread, costly event handlers, unnecessary re-renders in frameworks like React, and third-party scripts competing for the main thread.
CLS: Cumulative Layout Shift (Visual Stability)
CLS measures the total amount of unexpected visual displacement that occurs during the life of the page. When an element moves on screen without the user initiating it (a jumping button, shifting text, an ad pushing content), the CLS score increases. High CLS causes users to click the wrong element or lose their reading flow.
| CLS threshold | Rating | User experience |
|---|---|---|
| ≤ 0.1 | ✅ Good | Stable page, no visual jumps |
| 0.1 – 0.25 | ⚠️ Needs improvement | Visible shifts that interrupt reading |
| > 0.25 | ❌ Poor | Unstable page, high rate of misclicks |
The most common causes of high CLS: images and videos without defined width/height dimensions, ads or embeds without reserved space, web fonts that change the layout when loading (FOUT), and dynamically injected content inserted above existing content.
How do Core Web Vitals affect SEO?
Google uses Core Web Vitals as a ranking signal within what it calls "Page Experience." It is not the most powerful signal — content relevance remains the primary factor — but it can be a tiebreaker between two equally relevant pages. In competitive markets, poor CWV can cost you visible ranking positions.
- •Google applies the Page Experience signal at the individual URL level, not at the entire site level.
- •Pages with "good" CWV across all three metrics may gain a small ranking boost.
- •Pages with "poor" CWV do not receive a direct penalty, but may miss out on the boost that pages with good scores receive.
- •In Search Console you can see how many of your site's URLs are in "Good," "Needs improvement," or "Poor" status in the Page Experience report.
How to measure your Core Web Vitals
iRankly's Core Web Vitals Monitor analyzes up to 50 URLs at a time using the PageSpeed Insights API, which combines lab data (Lighthouse) with real field data (CrUX). Enter the most important URLs on your site to see which ones need priority attention.
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You can also monitor CWV directly in Google Search Console (the "Page Experience" and "Core Web Vitals" reports) with real data from your users, segmented by mobile and desktop.