What Are Core Web Vitals?
Core Web Vitals are three specific metrics that Google uses to measure how real visitors experience your website. They are not lab tests or simulated scores — they are aggregated measurements from actual Chrome browser users visiting your site over a rolling 28-day window.
Google uses these metrics as a ranking signal in search results. If your site fails Core Web Vitals in the field, it can affect your search visibility — and more importantly, it means your visitors are having a poor experience.
The Three Metrics
Largest Contentful Paint (LCP) measures how long it takes for your main content to appear on screen. This is typically your hero image, a large heading, or the first significant block of text. Google considers an LCP under 2.5 seconds “good” on mobile. Anything over 4 seconds is rated “poor.”
LCP is the metric most WordPress sites struggle with, especially on mobile devices where processors are slower and connections are weaker. Common causes include unoptimized hero images, render-blocking JavaScript, and slow server response times.
Interaction to Next Paint (INP) measures how quickly your site responds when a visitor clicks a button, taps a link, or types in a form field. It captures the delay between the interaction and the next visual update on screen. An INP under 200 milliseconds is “good.” Over 500 milliseconds is “poor.”
INP replaced the older First Input Delay (FID) metric in March 2024. It is a harder test to pass because it measures every interaction during a visit, not just the first one. Heavy JavaScript — from plugins, analytics scripts, and third-party widgets — is the most common cause of poor INP.
Cumulative Layout Shift (CLS) measures how much the page layout jumps around while loading. If you have ever tried to tap a button and the page shifted so you tapped something else instead, that is a layout shift. A CLS score under 0.1 is “good.” Over 0.25 is “poor.”
Layout shift is caused by images without dimensions, web fonts that swap in after text renders, and content injected dynamically by JavaScript. It is the most straightforward of the three metrics to fix.
Why Field Data Matters
Core Web Vitals are measured using field data — real measurements from real visitors using Chrome browsers. This is fundamentally different from running a lab test in PageSpeed Insights or Lighthouse.
A site can score 90 in a lab test and still fail Core Web Vitals in the field. The lab test runs on a simulated device with a stable connection. Field data reflects the actual devices, network conditions, and browsing patterns of your real audience.
The field data that Google uses for ranking comes from the Chrome User Experience Report (CrUX). You can see your site’s CrUX data in Google Search Console under the Core Web Vitals report. That is the definitive source — not your PageSpeed score.
How They Affect Search Rankings
Google incorporated Core Web Vitals into its Page Experience ranking signal in 2021. Sites that pass all three metrics do not get a dramatic ranking boost — but sites that fail them can lose ground to competitors who pass, especially in competitive search results where content quality is similar.
The practical impact: Core Web Vitals are a tiebreaker. When two pages have similar content relevance and authority, the one with better user experience metrics wins. For e-commerce sites competing on product and category pages, that tiebreaker matters.
Mochyon Lightspeed measures all three Core Web Vitals on your homepage using both field data (when available) and lab data, and explains each metric in plain English.
Further Reading
- Web Vitals (web.dev) — Google official guide to Core Web Vitals and the metrics that matter for user experience.
- Core Web Vitals and Search (Google Search Central) — How Google uses CWV as a ranking signal and what site owners need to know.