Core Web Vitals Explained: What LCP, CLS and TBT Mean for Your Revenue
Google's Core Web Vitals are more than a developer concern — they directly affect your search rankings and ad costs. Here's what each metric measures and why it matters.
What Core Web Vitals Actually Are
Core Web Vitals are three specific measurements Google uses to capture the real-world experience of loading and using a webpage. Unlike older metrics (like total page load time), they measure what visitors actually perceive — the speed of your main content appearing, how responsive the page feels, and whether the layout shifts unexpectedly.
Since 2021, they've been official Google ranking signals. Since 2022, they've been factored into Google's ad Quality Score. They're a direct lever on your search visibility and ad costs.
The Three Metrics Explained
Largest Contentful Paint (LCP)
LCP measures how long until the largest visible element on the page — usually your hero image or main heading — fully loads. Google's threshold is under 2.5 seconds. Above 4 seconds is rated poor. The most common causes: unoptimised images and slow server response times.
Total Blocking Time (TBT)
TBT measures the total time the browser is blocked by JavaScript during page load — the window when the page looks loaded but doesn't respond to clicks. Under 200ms is good. Third-party scripts are the primary culprit: analytics tags, chat widgets, and ad pixels all compete for main thread time.
Cumulative Layout Shift (CLS)
CLS measures visual instability — how much elements move around as the page loads. Under 0.1 is good. CLS is most damaging on mobile, where a layout shift can cause a visitor to tap the wrong element. It's also frequently cited in accessibility audits.
The Direct Revenue Connection
Google's research shows that as page load time increases from 1 to 3 seconds, the probability of a visitor leaving before interacting increases by 32%. At 5 seconds, it reaches 90%. Every percentage point of visitors who leave before the page loads is lost revenue — leads that never filled your form, products never added to a basket.
For businesses running Google Ads: poor Core Web Vitals reduce your Quality Score, meaning you pay more per click than a competitor with a faster site bidding the same amount.
How to Find Your Scores
Google's PageSpeed Insights will show your scores for any URL. A score of 90+ is good, 50–89 needs improvement, and below 50 is considered poor. A full 5-pillar audit combines your scores with a monthly ad spend overspend estimate — useful for prioritising the work with your developer or agency.
FREQUENTLY ASKED QUESTIONS
What are Core Web Vitals?+
Core Web Vitals are three specific performance metrics Google uses to measure user experience: Largest Contentful Paint (LCP, measures loading speed), Total Blocking Time (TBT, measures interactivity), and Cumulative Layout Shift (CLS, measures visual stability). They have been Google ranking signals since 2021.
What is a good Core Web Vitals score?+
Google rates each metric as Good, Needs Improvement, or Poor. For LCP: Good is under 2.5s, Poor is above 4s. For TBT: Good is under 200ms, Poor is above 600ms. For CLS: Good is under 0.1, Poor is above 0.25. A PageSpeed Insights score of 90+ indicates all three are in the Good range.
Do Core Web Vitals affect Google Ads costs?+
Yes. Google uses Core Web Vitals as part of the landing page experience component of Quality Score in Google Ads. Poor scores reduce your Quality Score, which means you pay more per click. A site moving from a score of 45 to 85 can reduce Google Ads CPCs by 30–60%.
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