What Core Web Vitals are and why they matter

In 2021, Google introduced Core Web Vitals as an official ranking signal — meaning your scores on these three metrics directly influence where your site appears in search results. They're part of a broader set of signals Google calls "page experience," which also includes mobile-friendliness and HTTPS security.

What makes Core Web Vitals different from older performance metrics is that they're based on real user data, not just lab measurements. Google collects anonymised performance data from Chrome users and uses it to assess how your site actually performs for real visitors — not just in ideal conditions.

The three metrics are Largest Contentful Paint (LCP), Interaction to Next Paint (INP), and Cumulative Layout Shift (CLS). Each one measures a different aspect of the user experience.

The three Core Web Vitals explained

Largest Contentful Paint (LCP) — loading performance

LCP measures how long it takes for the largest visible element on the page to fully load. This is usually a hero image, a large heading, or a video thumbnail. It's the metric most closely tied to the user's perception of how fast a page loads.

LCP thresholds:

Good: under 2.5 seconds
Needs improvement: 2.5–4.0 seconds
Poor: over 4.0 seconds

The most common causes of a poor LCP score are large unoptimised images, slow server response times, and render-blocking JavaScript or CSS that delays the page from displaying content.

Interaction to Next Paint (INP) — interactivity

INP measures how quickly your page responds to user interactions — clicks, taps, and keyboard inputs. It replaced an older metric called First Input Delay (FID) in March 2024. Where FID only measured the first interaction, INP measures the overall responsiveness throughout the entire page visit.

INP thresholds:

Good: under 200 milliseconds
Needs improvement: 200–500 milliseconds
Poor: over 500 milliseconds

Poor INP is most commonly caused by heavy JavaScript execution — particularly third-party scripts like analytics, chat widgets, and marketing tools that compete for the browser's processing time.

Cumulative Layout Shift (CLS) — visual stability

CLS measures how much the page layout shifts unexpectedly while loading. You've experienced this when you go to click a button and the page jumps just as you tap, sending you somewhere you didn't intend. CLS quantifies how often and how severely this happens.

CLS thresholds:

Good: under 0.1
Needs improvement: 0.1–0.25
Poor: over 0.25

The most common causes of poor CLS are images without defined width and height attributes, ads or embeds that load without reserved space, and web fonts that cause text to reflow as they load.

How to check your Core Web Vitals scores

There are two main ways to check your scores, and they can give different results — which is worth understanding.

PageSpeed Insights

Google's free PageSpeed Insights tool (pagespeed.web.dev) shows both lab data (simulated) and field data (real user measurements from Chrome). Enter your URL and look at the "Field Data" section at the top — this is what Google actually uses for ranking purposes. The "Lab Data" section below is useful for diagnosing issues but doesn't directly reflect your ranking signal.

Google Search Console

If you have Google Search Console set up for your site (which you should), the Core Web Vitals report under "Experience" shows which pages are passing, need improvement, or are failing — based on real user data aggregated over 28 days. This is the most authoritative view of how Google sees your site's performance.

Note that Search Console only shows data for pages with sufficient traffic. New or low-traffic pages may not appear in the report.

The most common fixes for each metric

Improving LCP

→ Compress and resize your hero image — this is the single most impactful fix for most sites
→ Use a content delivery network (CDN) to serve assets from servers closer to your visitors
→ Add fetchpriority="high" to your LCP image so the browser loads it first
→ Upgrade to faster hosting if your server response time (TTFB) is above 600ms

Improving INP

→ Audit and remove third-party scripts you don't actively use
→ Defer non-critical JavaScript so it loads after the page is interactive
→ Break up long JavaScript tasks into smaller chunks

Improving CLS

→ Add explicit width and height attributes to all images and video embeds
→ Reserve space for ads and dynamic content before they load
→ Use font-display: optional or preload your web fonts to prevent text reflow

The short version

1. Core Web Vitals are three metrics: LCP (loading), INP (interactivity), CLS (visual stability)
2. Google uses them as a ranking signal — poor scores can suppress your search visibility
3. Check your scores at pagespeed.web.dev and in Google Search Console
4. LCP target: under 2.5s · INP target: under 200ms · CLS target: under 0.1
5. Unoptimised images and excess third-party scripts are the most common causes of failure

What are Core Web Vitals?