What are Core Web Vitals?
Core Web Vitals are three metrics defined by Google to describe what a visitor experiences on a page: loading speed, responsiveness and visual stability. They are measured on real Chrome users, then summarized per page and per site.
On an online store, the three metrics map to three concrete moments: the product photo that is slow to appear, the “Add to cart” button that does not react right away, and the banner that shifts the page just as you are about to click.
LCP, INP, CLS: what thresholds should you meet?
| Metric | What it measures | Good | Needs improvement | Poor |
|---|---|---|---|---|
| LCP (Largest Contentful Paint) | Time to render the largest visible element | ≤ 2.5 s | 2.5 to 4 s | > 4 s |
| INP (Interaction to Next Paint) | Delay between an interaction and the visible response | ≤ 200 ms | 200 to 500 ms | > 500 ms |
| CLS (Cumulative Layout Shift) | Extent of unexpected layout shifts | ≤ 0.1 | 0.1 to 0.25 | > 0.25 |
The 75th percentile means three visits out of four must do at least as well as the threshold. A page that is fast on your fiber connection can therefore fail: your visitors on mobile data determine the result.
The Core Web Vitals assessment is passed only if all three metrics are in the “good” range. A single metric needing improvement is enough to fail it.
Field data vs lab data: what is the difference?
| Field data | Lab data | |
|---|---|---|
| Origin | Real Chrome visitors, last 28 days | One simulated load, at test time |
| Used by Google | Yes, for the assessment | No, it is for diagnosis |
| INP available | Yes | No: there is no real interaction |
| Availability | Only if the page gets enough visits | Always |
The first section of the PageSpeed Insights report, the one about real users, is what you copy into the checker. The performance score out of 100 shown further down is a lab grade: useful for finding what to fix, but not what Google uses.
A new or low-traffic store does not always have field data. In that case, rely on lab measurements for LCP and CLS, and come back to measure INP once traffic allows it.
How to test your store's Core Web Vitals
- 1
Pick a page that matters
A product or category page, not just the home page: those are the pages that receive Google Shopping and search traffic.
- 2
Run the measurement
Enter the address and open the PageSpeed Insights report. Stay on the Mobile tab, the most demanding one.
- 3
Copy LCP, INP and CLS
Copy the three real-user values into the checker.
- 4
Read the verdict
The checker grades each metric and shows store-specific fixes for those that are not “good”.
- 5
Repeat on another template
Product page, category, home, cart: each theme template has its own weaknesses.
Why do Core Web Vitals matter for e-commerce?
- The shopping experience — A slow image, an unresponsive button or a jumping page are friction at the worst moment: when the visitor is deciding to buy.
- Organic search — Google uses Core Web Vitals in its ranking systems. It is one signal among many: it does not make up for weak content, but it separates comparable pages.
- Advertising — A slow landing page wastes paid clicks. Before raising a budget, check that the page it sends traffic to loads properly on mobile.
Core Web Vitals do not get a Google Merchant Center account suspended. A product page that is unreachable or does not render for Google's crawler does: it is one of the points in the Google Merchant Center compliance checklist.
How to improve a store's LCP, INP and CLS
- LCP: the main image — On a product page the largest element is almost always the photo. Serve it in WebP or AVIF, at the right size, without lazy loading, and preload it.
- INP: app JavaScript — On Shopify as on WooCommerce, every app or plugin adds scripts. Remove the ones you no longer use and delay chat, reviews and tracking tools.
- CLS: reserved space — Give images dimensions, reserve space for cookie and promotion banners, and preload your fonts so text does not change size.
Fix in this order: the “poor” metric first, then those that “need improvement”. After a fix, lab data changes right away, but field data takes up to 28 days to fully reflect the change.
Where to track your Core Web Vitals over time
- PageSpeed Insights — To test a specific page on demand, with field data and a lab diagnosis.
- Google Search Console — The Core Web Vitals report groups your URLs by status and issue type, on mobile and desktop.
- Chrome DevTools — To reproduce an issue and verify a fix before shipping it.
A fast page is not enough if it is unclear to Google: also check your titles with the product title scorer and your search snippets with the meta description checker.
After speed, your store's compliance
Performance improves what your visitors experience. Compliance decides whether your products are allowed to show on Google Shopping. GClimb analyses your store's pages and lists what could justify a Google Merchant Center suspension: run the free audit.
Frequently asked questions
What are the three Core Web Vitals?
LCP (Largest Contentful Paint) measures loading speed, INP (Interaction to Next Paint) measures responsiveness, and CLS (Cumulative Layout Shift) measures visual stability. The “good” thresholds are 2.5 seconds, 200 milliseconds and 0.1.
Did INP replace FID?
Yes. INP replaced FID (First Input Delay) as a Core Web Vital in March 2024. FID only measured the delay of the first interaction; INP takes every interaction during the visit into account.
Are Core Web Vitals a Google ranking factor?
Google says it uses Core Web Vitals in its ranking systems. It is one signal among many: content relevance remains decisive, and good Core Web Vitals do not guarantee a better position.
Why does PageSpeed Insights show no field data for my page?
Field data only exists if the page gets enough visits from real Chrome users. For a low-traffic page, PageSpeed Insights shows data for the whole site, or lab data only.
My performance score is 90 but the assessment fails. Why?
The score out of 100 is a lab grade, calculated on a simulated load. The Core Web Vitals assessment relies on real visits: if your visitors have slower devices or connections than the simulation, it can fail despite a good score.
Should I test on mobile or desktop?
Both are assessed separately. Start with mobile: it usually accounts for most of a store's visits and is where good results are hardest to reach.
How long does it take to see the effect of a fix?
Lab data changes immediately. Field data is calculated over a rolling 28-day window: the improvement shows up gradually and is only complete at the end of that period.