Core Web Vitals checker

It runs Google's PageSpeed Insights API from your browser for the URL you enter. The three field figures come from the Chrome User Experience Report, which aggregates real visits over the previous 28 days; they are the numbers Google uses for page experience, and they exist only when a page has enough traffic. The lab figures come from a single simulated load and vary between runs; they are useful for diagnosing what to fix, not for pass or fail decisions.

Largest Contentful Paint is good at 2.5 seconds or less, measured at the 75th percentile of visits. Interaction to Next Paint is good at 200 milliseconds or less. Cumulative Layout Shift is good at 0.1 or less. A page passes when all three are good at the 75th percentile. Product and collection pages matter more than the homepage here, because they receive most of the traffic from search and ads and they are where the theme's weight, image handling and app scripts do the most damage.

For LCP, the hero or product image: its size, format, whether it is preloaded, and whether the theme lazy-loads it by mistake. For INP, third-party scripts and app embeds competing for the main thread when the shopper taps a variant or the add-to-cart button. For CLS, images and embeds without reserved dimensions, banners injected after load, and fonts that swap late. Check a product page, a collection page and the homepage separately, on mobile first.