Core Web Vitals are three metrics that Google uses to evaluate the user experience of a web page. They measure loading performance, interactivity, and visual stability. Unlike vanity metrics, these numbers directly correlate with how users perceive your site.
Largest Contentful Paint
LCP measures how long it takes for the largest visible element to render. This is usually a hero image, a headline, or a video thumbnail. Good LCP is under 2.5 seconds. Anything above 4 seconds is poor.
The most common causes of slow LCP are unoptimized images, render-blocking resources, and slow server response times.
Our approach: serve images in modern formats with proper sizing, preload the LCP image in the document head, and ensure the server responds in under 200 milliseconds. On static sites built with Astro, the server response time is essentially zero because we serve from a CDN.
Interaction to Next Paint
INP replaced First Input Delay in March 2024. It measures the responsiveness of every interaction on the page — clicks, taps, and key presses — not just the first one.
Good INP is under 200 milliseconds. This means that from the moment a user taps a button to the moment the browser paints the result, less than a fifth of a second should pass.
Heavy JavaScript is the primary cause of poor INP. Long tasks on the main thread block the browser from responding to user input. Every third-party script, analytics tag, and animation library contributes to the problem.
Our approach: minimize client-side JavaScript, break long tasks into smaller chunks, and defer non-essential scripts. On content sites, we target zero JavaScript on the main thread for non-interactive pages.
Cumulative Layout Shift
CLS measures visual stability. If elements on the page move after the initial render — text jumps down when an ad loads, buttons shift when a font swaps, content pushes around when images load — the CLS score increases.
Good CLS is under 0.1. Users hate layout shifts because they cause misclicks, lost reading position, and a general feeling that the page is unstable.
Our approach: always set explicit dimensions on images and videos, use font-display swap with size-adjusted fallback fonts, and avoid injecting content above existing content after the initial render.
Measuring in the field
Lab tools like Lighthouse and WebPageTest give you synthetic scores under controlled conditions. These are useful for development but do not reflect real user experience.
Field data from the Chrome User Experience Report or a real user monitoring tool shows how actual visitors experience your site on their devices, their networks, and their browsers. We have seen sites score 95 on Lighthouse but fail Core Web Vitals in the field because real users were on slower devices than the testing machine.
We include real user monitoring on every project and review the data monthly. Regressions show up in field data before anyone files a bug report.
The business impact
Faster sites convert better. This is not opinion — it is documented across hundreds of case studies. A 100-millisecond improvement in LCP can increase conversion rates by measurable percentages.
Core Web Vitals also affect search ranking. Google has confirmed that page experience is a ranking factor, with CWV as the technical foundation.
For our clients, the argument is straightforward: investing in performance is investing in revenue. Every millisecond saved is a user retained.
