Core Web Vitals
Definition
Core Web Vitals are three metrics Google uses to measure real-user page experience: Largest Contentful Paint (loading), Interaction to Next Paint (responsiveness), and Cumulative Layout Shift (visual stability). Google publishes a good threshold for each and uses the results as a ranking signal. What counts is field data from real visitors, not lab scores.
Why it matters for your store
Speed is conversion. A product page that takes four seconds on a mobile connection loses visitors before it loads, and those visitors never appear in your funnel. Core Web Vitals turn that into three numbers you can track and a threshold you can aim for.
They are also a ranking input. Between two pages with similar content, the one with better vitals tends to rank higher. And because the metrics are collected from real visitors in Chrome, a store cannot pass them in a lab and fail them in the field; the field is what is measured.
Most vitals problems come from the platform and the theme: server response time, image handling, script weight, and layout behaviour. The merchant's content rarely changes them much.
How Kambloo handles it
The storefront has a performance budget that is stricter than Google's thresholds: LCP under 2.0 seconds on mobile at the 75th percentile, CLS under 0.1, INP under 200 milliseconds, plus caps on JavaScript (90 KB compressed), CSS, and above-the-fold image weight. The budget is measured in the build pipeline with Lighthouse, and a change that exceeds it cannot be merged.
The budget is met by design: server-rendered pages with full-page caching for anonymous visitors, a CDN in front of every store, images pre-generated in AVIF and WebP at five sizes with width and height always set, one precompiled CSS file, and third-party scripts deferred. Themes are checked against the same budget before they are accepted.
A lightweight measurement snippet collects real-user LCP, INP, and CLS per store and feeds the SEO health report in the admin panel.
Questions about Core Web Vitals
What are the good thresholds for Core Web Vitals?
LCP of 2.5 seconds or less, INP of 200 milliseconds or less, and CLS of 0.1 or less, each measured at the 75th percentile of page visits.
Can a theme break Core Web Vitals?
Yes. Heavy scripts, unsized images, and web fonts without fallbacks are the usual causes. Kambloo checks themes against a performance budget before they are accepted.