The Current Landscape of Core Web Vitals
Core Web Vitals are a set of metrics defined by Google to measure user experience on web pages. In March 2024, Google finalized the announced transition: INP (Interaction to Next Paint) officially replaced FID (First Input Delay) as the third metric in the set.
For e-commerces, this change has direct implications. FID measured only the latency of the first interaction. INP measures the responsiveness of all interactions during the page's lifetime — filter clicks, add to cart, modal opens, scroll. A store with good FID may have poor INP and now this matters to Google.
INP: Why It Is Harder to Control in E-commerce
INP records the 98th percentile latency of all user interactions with the page. For an e-commerce store, this includes: category filter click, product size/color click, add to cart, gallery open, chat widget interaction.
What makes INP difficult in e-commerce is that each app and widget adds JavaScript that competes for the main thread. A live chat script loaded synchronously can block the response to an "add to cart" click by 500ms+ — which INP will capture as a poor interaction.
Real INP problem: Stores with many Shopify apps, WooCommerce plugins or Magento modules frequently have poor INP even when LCP is within the threshold. Diagnosis requires main thread analysis with Chrome DevTools or WebPageTest.
LCP: Still the Most Critical Metric for E-commerce
Despite the INP change, LCP remains the metric with the greatest direct impact on conversion. For e-commerce stores, the LCP element is almost always the hero image of the home page or the main product image.
What changed in 2025 regarding LCP is the precision with which Google measures field data (CrUX). Even if your Lighthouse shows a good LCP in the lab, if real user data shows poor LCP, it is the field data that will weigh on ranking.
How to Improve LCP by Platform
| Platform | Most common cause of high LCP | Priority solution |
|---|---|---|
| Shopify | Hero image without fetchpriority + blocking apps | fetchpriority="high" + app audit |
| Magento | FPC disabled or no Varnish + images without CDN | Enable FPC + configure Varnish + CDN |
| WooCommerce | Heavy theme + plugins without lazy load + slow hosting | Optimize theme + VPS hosting + image CDN |
CLS: The Invisible Problem of Stores with Banners and Pop-ups
Cumulative Layout Shift measures how much the visual content "jumps" during loading. In e-commerces, the biggest CLS causes are: cookie banners that appear after content, discount pop-ups that push content, product images without defined dimensions and review iframes that expand when loading.
Most effective fix: Always define width and height on all images. Reserve space for banners and pop-ups with CSS min-height before loading them. Load third-party iframes asynchronously with reserved dimensions.
What to Prioritize in Your 2025 Audit
With three metrics to manage, the priority sequence for e-commerces is usually:
- 1st LCP — greatest impact on conversion and easiest to fix with image and cache optimizations
- 2nd INP — more complex, requires script analysis and may involve replacing apps
- 3rd CLS — more predictable, generally fixed with CSS adjustments and image sizing
Recommended tool: Use WebPageTest (webpagetest.org) with the "Web Vitals" option enabled. It shows a complete waterfall, identifies LCP elements, measures simulated INP and points to scripts that cause long tasks — all in a free interface.
Field Data vs. Lab Data
This distinction is fundamental to understanding why PageSpeed Insights may show a high score while Google Search Console points to Core Web Vitals problems.
Lab data (Lighthouse, PageSpeed Insights) simulates a user on a specific connection and device, under controlled conditions. They are reproducible and useful for technical diagnostics.
Field data (CrUX — Chrome User Experience Report) measures real users with different devices, connections and contexts. These are the data Google uses for ranking.
Your goal should be to improve field data — measured in Google Search Console under "Page Experience". Lighthouse is a diagnostic tool, not the final metric.
Want to know the real state of your store's Core Web Vitals?
I perform a complete LCP, INP and CLS audit with field and lab data. You receive a report with identified root cause and correction priorities — without unnecessary technical jargon.