Core Web Vitals are the three core metrics Google uses to measure page experience: LCP (Largest Contentful Paint) measures how quickly the page's main content loads, INP (Interaction to Next Paint) measures how quickly the page responds to user interactions, and CLS (Cumulative Layout Shift) measures how much elements shift while the page loads. For a good result, LCP should be under 2.5 seconds, INP under 200 milliseconds and CLS under 0.1. In Shopify stores, the three factors that most affect these metrics are the theme's structure, installed apps and images.
In this guide we explain the Core Web Vitals metrics, the optimisations Shopify does automatically, the most common speed issues and step-by-step improvement methods.
Core Web Vitals metrics
Metric | What it measures | Good | Needs improvement | Poor |
|---|---|---|---|---|
LCP | Load time of the main content | ≤ 2.5 s | 2.5–4 s | > 4 s |
INP | Response time to interactions | ≤ 200 ms | 200–500 ms | > 500 ms |
CLS | Amount of visual shift | ≤ 0.1 | 0.1–0.25 | > 0.25 |
INP replaced the old FID (First Input Delay) metric in March 2024. While FID only measured the first interaction, INP assesses all interactions throughout the page.
The difference between field data and lab data
Field data: data collected from real users' Chrome browsers (Chrome UX Report). This is the data taken into account by Google's ranking systems. The Core Web Vitals report in Search Console and the top section of PageSpeed Insights show field data.
Lab data: data produced by tools such as Lighthouse from a single simulation. It's useful for diagnosing issues but doesn't fully reflect real user experience.
A low PageSpeed score isn't a problem on its own; the real goal is for all three metrics to be in the "good" range in field data.
What Shopify does for you automatically
Global CDN: store files and images are served from the server closest to the user.
Image optimisation: Shopify can automatically serve images in modern formats and different sizes.
Hosting and scaling: server infrastructure and traffic spikes are managed by Shopify.
Web performance report: the performance report in the Shopify admin shows your store's Core Web Vitals values based on real user data.
Thanks to this infrastructure, most speed issues in Shopify stores come not from the server but from the theme, apps and content. We explain the hosting side in our Shopify hosting article.
The issues that most affect speed in Shopify stores
1. Theme structure
Themes that load many sections, animations and features at once also load unused code on every page. Heavy customisations on top of off-the-shelf themes also degrade performance over time. We explain theme choice and the lightweight theme framework approach in our article on paid and free themes.
2. Apps and third-party code
Each app can add its own JavaScript and CSS files to the store. Leftovers that deleted apps leave in the theme code also keep slowing the page down. Marketing tags, chat widgets and review apps are among the third-party code that most affects INP and LCP.
3. Images
Large, uncompressed or incorrectly sized images directly lengthen LCP. Images without specified dimensions increase CLS by shifting the page when they load.
Improving LCP
Prioritise the main image. Don't apply lazy loading to the first image on the homepage and product page; load it with high priority where possible.
Use correctly sized images. Serve images in sizes suited to the device (srcset) with Shopify's image filters.
Reduce sliders. Large sliders on the homepage are often the biggest cause of LCP; a single strong image often performs better.
Optimise fonts. Use a small number of font weights and make sure fonts load without blocking text.
Improving INP
Remove unused apps and clean up leftover code.
Reduce third-party tags. Clean up unnecessary tags in Google Tag Manager; don't use multiple tracking codes that do the same job.
Defer heavy JavaScript. Load non-critical elements such as chat widgets and pop-ups after the page is ready for interaction.
Test filters and variant selectors. Collection filters and variant changes are common places for INP issues.
Improving CLS
Give images and videos dimensions. Width and height information lets the browser reserve space for the image in advance.
Use dynamic banners carefully. Announcement bars and cookie windows that appear after the page loads push content down.
Balance font swaps. The size difference between the fallback font and the actual font can cause text to shift.
How should you measure performance?
Search Console Core Web Vitals report: shows problematic URL groups with real user data.
Shopify web performance report: lets you track your store's Core Web Vitals values and the impact of changes.
PageSpeed Insights: shows both field and lab data for a single page.
Chrome DevTools: the Performance tab lets you examine in detail which code is slowing the page down.
It can take up to 28 days for changes to show in field data, because the reports cover the last 28 days of user data.
Speed and conversion
A fast store matters not only for SEO but for conversion. Mobile users in particular quickly abandon slow-loading pages. We explain other areas of conversion optimisation in our conversion rate article.
Frequently asked questions
My PageSpeed score is low; will my rankings drop?
Google takes into account Core Web Vitals data collected from real users, not the lab score. Page experience is also a smaller factor than content quality. Still, good values matter for both user experience and conversion.
Should I install a speed optimisation app?
Speed apps can help in some cases, but most issues come from the theme and apps. Cleaning up unnecessary apps and code first, then moving forward by measuring, gives more reliable results.
Which theme is faster?
Shopify's current themes generally provide a good performance foundation. The real difference comes from customisations and apps added to the theme.
Let's improve your store's speed together
Improving Core Web Vitals supports both search visibility and conversion rate. If you'd like to review your Shopify store's performance together, get in touch through our Shopify technical support page.
