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Google's Core Web Vitals thresholds are a Largest Contentful Paint of 2.5 seconds or less, an Interaction to Next Paint of 200 milliseconds or less, and a Cumulative Layout Shift of 0.1 or less, measured at the 75th percentile of real visits. On most Shopify and custom stores the failures come from images, third-party scripts and fonts, in that order, and fixing those three is a few weeks of work rather than a replatform.
Updated · 4 min read
Largest Contentful Paint is how long the biggest thing above the fold takes to appear, usually the hero image or the product photo. Good is 2.5 seconds or less. Interaction to Next Paint is how long the page takes to respond visibly after a tap or click, across the whole visit; good is 200 milliseconds or less. It replaced First Input Delay in March 2024. Cumulative Layout Shift is how much the page jumps around while loading; good is 0.1 or less.
Google grades these from real Chrome visitors, at the 75th percentile, over a rolling 28 days. That matters: a Lighthouse score is a lab test on one simulated device and is not the grade. Field data in Search Console is.
Core Web Vitals are a ranking signal, but a small one, and chasing them for ranking alone is the wrong reason. The reason is conversion. The published studies vary in their numbers and agree in their direction: slower pages sell less, the effect is strongest on mobile, and mobile is where most store traffic is.
Speed is also the first impression a store makes on a visitor who arrived from an ad you paid for. A product page that takes four seconds to show the product has spent the click before the shopper saw anything.
Images, almost always. Hero and product images served at full resolution to a phone, in formats from 2010, lazy-loaded when they are the first thing on screen, or waiting behind a carousel app that loads twelve slides to show one. Fonts that block the headline until the font file arrives. Apps that inject scripts ahead of the content.
The fixes are well understood: modern formats such as WebP and AVIF at the sizes actually displayed, the above-the-fold image loaded eagerly and preloaded, everything below it lazy, and a font strategy that shows text immediately in a fallback. On Shopify that is theme work and app pruning, not a new platform.
Interaction delay comes from the main thread being busy, and on a store the thread is busy with third parties: chat widgets, review apps, trackers, A/B testing tools, each one a script the shopper did not ask for. A typical store runs somewhere between ten and thirty of them. Layout shift comes from images without dimensions, fonts swapping size, and banners or cookie bars that push the content down after it has rendered.
Fixes: audit every script and remove the ones that do not earn their place, defer the rest until after the page is interactive, put explicit dimensions on every image and embed, reserve space for anything that loads late, and tune the fallback font so the swap does not reflow the page.
Measure in the field first, with Search Console and the Chrome User Experience data, to know which pages fail and on which metric. Then the lab, with Lighthouse, to find the causes. Then the work, in order of return: the image pipeline, the third-party audit, the font strategy, and deferral of everything the first paint does not need.
For most stores that is a few weeks and gets the store passing. Re-measure after the 28-day window rolls over, because the field data lags the fix. Replatforming is the answer only when the platform itself is the bottleneck, which is rarer than agencies selling replatforms suggest; the headless and rebuild guides cover when it genuinely is.
Shopify's infrastructure is fast. Slow Shopify stores are slow because of theme apps, oversized images and script bloat, and a clean theme passes Core Web Vitals comfortably. The platform is rarely the problem.
No. Lighthouse is a lab test on one simulated device. Google grades the field data from real visitors. A store that scores 70 in the lab with passing field data is in better shape than one that scores 100 on a developer's laptop and fails on real phones.
Field data is a rolling 28-day window, so the improvement shows gradually and the full effect is visible about a month after the fix ships.
The primary documents the numbers in this guide come from.
Most stores should stay on a Shopify theme. Headless earns its cost when you need control Shopify's theme layer will not give you, and it charges you in build time, hosting, and a permanent front-end maintenance job. Here is the honest test.
EcommerceA Shopify migration for a catalog of a few hundred to a few thousand products typically takes 8 to 14 weeks, and the work that protects revenue is a complete 301 redirect map built before anything else. Customer passwords never migrate, Shopify sets your URL structure for you, and the old platform's bad data follows you unless it is cleaned first.
Ads and SEOPut the first dollar into Google Ads if you need customers this quarter and your site already converts; put it into SEO if your margins cannot carry a cost per click and you can wait 4 to 6 months for results. Most small businesses should run a small search campaign first, because it shows in weeks which searches are worth years of SEO.
Nockta builds ecommerce, custom software, and Amazon operations for businesses across Ontario and Quebec. See where we work or the full service list.