# Core Web Vitals and conversions: how page speed affects your conversion rate > What LCP, INP and CLS measure, how slow pages cost conversions, and how to measure the link on your own site before and after a speed fix. Source: https://cuttl.io/blog/core-web-vitals-and-conversions/ · Last updated: 2026-11-01 What LCP, INP and CLS measure, how slow pages cost conversions, and how to measure the link on your own site before and after a speed fix. ## What Core Web Vitals are Core Web Vitals are Google’s three measures of how a page feels to real visitors: Largest Contentful Paint (LCP) for loading, Interaction to Next Paint (INP) for response to taps and clicks, and Cumulative Layout Shift (CLS) for visual stability. A page passes when, at the 75th percentile of page loads, LCP is 2.5 seconds or less, INP is 200 ms or less and CLS is 0.1 or less. Each metric has three bands. Above 4 seconds of LCP, 500 ms of INP or 0.25 of CLS counts as poor, and the range in between needs improvement. INP replaced First Input Delay as a Core Web Vital on 12 March 2024, so older articles that quote FID are out of date. ## How each metric costs conversions Each Core Web Vital fails visitors at a different moment, so each one hurts a different step of the conversion. - Slow LCP means the main image, headline or offer appears late. On a landing page from a paid ad, visitors who see a blank or half-drawn page for four seconds often go back to the search results before they read anything. - Slow INP means the page reacts late to a tap. A visitor taps Add to cart, nothing seems to happen, and they tap again, or give up. Repeated taps on a slow button often show up as rage clicks. - High CLS means content moves while people read or tap. A late banner pushes the page down and the tap meant for Checkout lands on a link to another page. Core Web Vitals are also part of Google’s page experience signals for search ranking, a smaller factor than relevance and content. ## How to measure the link on your own site To see what speed costs your site, compare the conversion rate of sessions that loaded fast with sessions that loaded slowly, on the same pages and devices. Industry figures tell you speed matters in general. Your own data tells you whether it matters on your checkout this month. Four rules keep the comparison fair: - Use field data from real visitors. Lab tools such as Lighthouse test one load on one simulated device, which is useful for debugging but says nothing about conversions. - Group by page template, such as all product pages, and split by device. A slow mobile product page and a fast desktop home page shouldn’t share one average. - Put sessions in speed bands, for example LCP under 1.5 s, 1.5 to 2.5 s, 2.5 to 4 s and over 4 s, and look at the conversion rate and sample size of each band. - Read the gap as a correlation. Slow sessions also come from older phones, weaker networks and other countries, and those visitors may convert less for other reasons too. If the over 4 s band converts at half the rate of the good band on mobile product pages, with a few hundred sessions in each, that template is worth a closer look. Site health in Journey Insights. See how Cuttl handles Core Web Vitals. ## Which pages to fix first Fix the slow pages that sit closest to a conversion and get the most traffic: product, pricing, signup and checkout pages, and the landing pages your ads send people to. A blog post that loads in 3 seconds matters less than a checkout step that does. Most speed problems live in a template, so one fix covers every page built from it. Common causes by metric: - LCP: a large hero image that isn’t compressed, sized or preloaded; slow server responses; fonts or scripts that block the first paint. - INP: long JavaScript tasks on the main thread, often from third-party scripts, heavy event handlers or large page updates after a tap. - CLS: images and embeds without set dimensions, banners and cookie notices that push content down, and web fonts that swap in at a different size. Before you start, note each template’s 75th percentile values and its conversion rate, so you have a baseline to compare against. ## How to confirm a speed fix raised conversions A speed fix is a change like any other, so measure its effect on conversions instead of assuming it. The two usual methods are a before-and-after comparison and an A/B test. - Before-and-after analysis compares the conversion rate in the weeks before the fix with the weeks after it. It needs no traffic split, so it suits sites with little traffic, but seasonality and campaigns can blur the result. - An A/B test serves the fast version to half of visitors and the current version to the other half, which separates the speed effect from everything else. In a case study on web.dev, Rakuten 24 tested Core Web Vitals work this way on a landing page and reported a 33.13% higher conversion rate. Check the vitals themselves too. If LCP at the 75th percentile hasn’t moved after the fix, a change in conversion rate came from something else. ## Core Web Vitals in Cuttl Cuttl measures LCP, INP and CLS in every real session through the Cuttl Tag and shows them by page group and device in Journey Insights → Site health, on every plan. - The Speed view shows the 75th percentile of each metric and, on Pro and Max, the conversion rate in each speed band, with sample sizes and 95% intervals, adjusted for device. - Cost of slowness estimates the conversions lost each month if slow sessions had converted like the good ones. Cuttl labels it as a correlation, for the reasons above. - A page group whose LCP is over 4 seconds at the 75th percentile becomes a Slow page issue, next to rage clicks, dead clicks and errors, with the affected sessions. - Log a speed fix as a change, and Change Impact compares conversions before and after it, with the speed baseline saved on the change. To test the fix against the current version, run it as an A/B test in Web Experiments. Bots and AI agents we detect are left out of these numbers. ## Common questions ### What are the 3 Core Web Vitals? Largest Contentful Paint (LCP) for loading, Interaction to Next Paint (INP) for responsiveness and Cumulative Layout Shift (CLS) for visual stability. ### What is a good Core Web Vitals score? At the 75th percentile of page loads: LCP of 2.5 seconds or less, INP of 200 ms or less and CLS of 0.1 or less. A page passes when all three are good. ### Do Core Web Vitals affect conversion rate? Often, yes. Sessions with slow pages tend to convert less, but the size of the effect varies by site, so measure it on your own pages and confirm fixes with a before-and-after comparison or an A/B test. ### Are Core Web Vitals a ranking factor? Yes, as part of Google’s page experience signals, but a small one next to relevance and content quality. ### What replaced First Input Delay? Interaction to Next Paint (INP) replaced First Input Delay as a Core Web Vital on 12 March 2024. INP looks at every interaction on the page, not only the first. ### What is form analytics? Find the field where visitors give up ### What is bounce rate? How it’s measured and what to do about it ### What is a conversion funnel?