Core Web Vitals Explained: LCP, CLS, and INP in Plain English
Core Web Vitals aren't one score — they're three specific, measurable things: how fast the main content loads, whether the page jumps around while loading, and how quickly it responds to a click. Here's what each one actually measures.
Three specific, measurable things
"Core Web Vitals" isn't one vague performance score — it's three named metrics, each measuring something distinct and specific about how a page feels to use:
- LCP (Largest Contentful Paint) — how long it takes for the biggest visible element ( usually a hero image or main heading) to finish loading.
- CLS (Cumulative Layout Shift) — how much visible content jumps around unexpectedly while the page loads, like text shifting down when a late-loading image or ad claims space above it.
- INP (Interaction to Next Paint) — how quickly the page visibly responds after a click or tap. INP replaced an older metric called FID in 2024 — if you've seen FID referenced in older articles, INP is its successor and measures responsiveness more comprehensively.
What counts as "good"
- LCP: 2.5 seconds or less is good. 2.5–4 seconds needs improvement. Above 4 seconds is poor.
- CLS: 0.1 or lower is good. 0.1–0.25 needs improvement. Above 0.25 is poor.
- INP: 200 milliseconds or less is good. 200–500ms needs improvement. Above 500ms is poor.
CheckSEO's Core Web Vitals Checker runs a real Lighthouse test and reports actual LCP, CLS, and INP for any URL — no signup.
Test your Core Web VitalsWhat usually causes a poor score
Oversized, unoptimized images
The single most common cause of a poor LCP — a multi-megabyte hero image takes visibly longer to paint than a properly compressed and correctly sized one.
Render-blocking CSS and JavaScript
Scripts and stylesheets that must fully load before the browser can paint anything delay LCP even when the actual content is small and fast to download on its own.
Images and ads without reserved space
The leading cause of poor CLS — when an image or ad slot has no declared width and height, the browser doesn't reserve space for it, so surrounding content jumps the moment it finishes loading.
Heavy JavaScript running on interaction
The leading cause of poor INP — a click handler that runs expensive processing before updating the screen makes the page feel unresponsive even if the initial load was fast.
Slow server response time
Every other metric waits on the initial response from the server. A slow backend delays LCP indirectly no matter how well-optimized the front end is.
Lab data versus field data
A Lighthouse-style test measures lab data — a simulated run under controlled conditions, useful for debugging because it's consistent and repeatable. Google's Chrome UX Report (CrUX) measures field data — real measurements from actual Chrome users visiting the page, which is what Google's ranking systems actually reference. The two can disagree: a page might test well in a lab simulation but perform worse for real visitors on slower connections or older devices. Checking both gives a fuller picture than either alone.
Test your own page
CheckSEO's Core Web Vitals Checker runs a real Lighthouse check and reports lab data alongside real-world Chrome UX Report field data when it's available for the URL.
Frequently asked questions
What are the Core Web Vitals?
Three metrics Google uses to measure real-world page experience: LCP (how long the largest visible element takes to load), CLS (how much the layout shifts unexpectedly while loading), and INP (how quickly the page responds after a click or tap).
What is a good LCP score?
2.5 seconds or less is considered good. Between 2.5 and 4 seconds needs improvement, and anything above 4 seconds is classified as poor.
What is a good CLS score?
0.1 or lower is good. This measures how much visible content shifts position unexpectedly — commonly caused by images or ads that load without reserved space, pushing text down after a visitor has already started reading.
What causes a poor LCP score?
Most often oversized, unoptimized images; render-blocking CSS or JavaScript that delays the main content from painting; and slow server response times, since every other metric waits on the initial response.
How can I improve my LCP score?
Compress and correctly size images, especially any large hero image or heading graphic; eliminate render-blocking resources in the page's critical path; and improve server response time — the three levers that account for most real-world LCP problems.
Check your own site with the Core Web Vitals Checker.
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