How to Use This Core Web Vitals Checklist
Core Web Vitals measure three user experience signals that Google uses as ranking factors: Largest Contentful Paint (LCP), Interaction to Next Paint (INP), and Cumulative Layout Shift (CLS). For ecommerce stores, poor scores translate directly into lost conversions and reduced organic visibility. This checklist gives you 12 specific audit items, each with a defined pass threshold and a fail condition, so every check produces a binary outcome you can act on.
Run this audit using Google PageSpeed Insights, Chrome DevTools, and the Core Web Vitals report in Google Search Console. Prioritize field data (real user measurements) over lab data because Google's ranking signal is based on the Chrome User Experience Report (CrUX), which reflects actual visitors. If your store has low traffic and CrUX data is unavailable, lab scores from PageSpeed Insights are the next best proxy.
LCP Checks: 4 Items to Audit
Check 1 — LCP Score. Open PageSpeed Insights for your homepage, a top category page, and a top product page. PASS: LCP is 2.5 seconds or under on mobile. FAIL: LCP exceeds 2.5 seconds on any of the three page types. Each page type earns a separate pass/fail.
Check 2 — LCP Element Is a Preloaded Hero Image. In Chrome DevTools > Performance panel, identify the LCP element. PASS: The LCP element is an image with a <link rel='preload'> tag in the <head> and fetchpriority='high' on the <img> tag. FAIL: The preload tag is absent, the LCP element is a background-image CSS property (not preloadable), or fetchpriority is missing.
Check 3 — LCP Image Uses a Next-Gen Format and Correct Sizing. PASS: The LCP image is served as WebP or AVIF, and its intrinsic dimensions match its rendered dimensions within 10%. FAIL: The image is JPEG or PNG, or it is more than 20% larger than its rendered size, wasting bytes that delay LCP.
Check 4 — Time to First Byte (TTFB) Under 600 ms. PASS: TTFB measured in PageSpeed Insights is under 600 ms. FAIL: TTFB exceeds 600 ms. Slow TTFB — caused by unoptimized server response, no CDN, or heavy server-side rendering — is the single most common root cause of LCP failures on ecommerce platforms.
INP Checks: 4 Items to Audit
Check 5 — INP Score. PASS: INP is under 200 ms on mobile in field data from Search Console or CrUX. FAIL: INP is between 200–500 ms (needs improvement) or above 500 ms (poor). INP replaced First Input Delay in March 2024 and measures the full responsiveness of every interaction, not just the first one.
Check 6 — No Long Tasks Blocking the Main Thread on Product Pages. In Chrome DevTools > Performance panel, record a page load and interaction simulation. PASS: No JavaScript task exceeds 50 ms on the main thread during or after page load. FAIL: Tasks longer than 50 ms appear, indicating JavaScript that delays the browser's ability to respond to clicks, especially critical on add-to-cart buttons.
Check 7 — Third-Party Scripts Are Loaded with defer or async. Inspect the page source or use PageSpeed Insights > Opportunities. PASS: All non-critical third-party scripts (chat widgets, analytics, ad pixels) use defer or async attributes. FAIL: Any render-blocking third-party script appears in the critical path. On ecommerce stores, tag managers and review widgets are the most common offenders.
Check 8 — Input Handlers Do Not Run Expensive Synchronous Work. Use the Interaction section in Chrome DevTools > Performance Insights to record a click on the add-to-cart button. PASS: The input handler completes within 50 ms and does not trigger synchronous DOM reads followed by writes (layout thrashing). FAIL: The handler duration exceeds 50 ms or shows interleaved reads and writes in the flame chart.
CLS Checks: 4 Items to Audit
Check 9 — CLS Score. PASS: CLS is 0.1 or under across homepage, category, and product pages in field data. FAIL: CLS exceeds 0.1 on any page type. A score above 0.25 is classified as poor by Google. CLS is measured as the sum of all unexpected layout shifts, weighted by impact fraction and distance fraction.
Check 10 — All Images and Video Embeds Have Explicit Width and Height Attributes. Inspect the HTML of product images, banner images, and embedded videos. PASS: Every image and video has both width and height attributes set in the HTML, allowing the browser to reserve space before the asset loads. FAIL: Any image or video is missing either attribute, which causes the page to reflow when the asset arrives.
Check 11 — No Layout Shifts From Late-Loading Fonts. Use the CLS debug tool in PageSpeed Insights or the Layout Shift Regions overlay in Chrome DevTools. PASS: Web fonts use font-display: swap or optional and are preloaded in the <head>, so no text block shifts when custom fonts load. FAIL: Text shifts position after initial render, indicating fonts are loading without a preconnect or preload hint and without a stable fallback.
Check 12 — Cookie Banners and Promotional Bars Do Not Inject Above Existing Content. Manually load the page in an incognito window and observe the first 5 seconds. PASS: The cookie consent banner and any promotional announcement bar are either pre-rendered server-side in a reserved space or animate in from an overlay that does not push content down. FAIL: A banner appears after the page renders and pushes the hero image or navigation downward, creating a visible shift.
Prioritizing Fixes After the Audit
Score every check as pass or fail, then group failures by metric. LCP failures on product pages have the highest revenue impact because product pages drive purchase intent traffic. Fix TTFB and image preloading first since those two items resolve the majority of LCP failures without requiring a platform migration or extensive development work.
INP failures are most damaging on mobile, where CPU constraints amplify long tasks. Audit your JavaScript bundle on add-to-cart and checkout flows first — those interactions are highest-stakes. CLS failures from images and cookie banners are typically the fastest to fix: adding width and height attributes to image tags is a one-time template change that resolves the issue across all product pages simultaneously.
Recheck all 12 items after deploying fixes, using both PageSpeed Insights and the Core Web Vitals report in Google Search Console. Field data in Search Console lags 28 days, so expect to wait four weeks before CrUX data reflects your improvements. Track lab data in PageSpeed Insights weekly in the interim to confirm fixes are holding.