If you have run a PageSpeed Insights test in the last two years, you have seen the three Core Web Vitals scores at the top of the report: Largest Contentful Paint, Interaction to Next Paint, and Cumulative Layout Shift. But what do these metrics actually measure, and why should you care about them?
Largest Contentful Paint (LCP)
LCP measures how long it takes for the largest visible element on the page to render — typically a hero image, a heading, or a video poster. Google wants this to happen in under 2.5 seconds. If your LCP is above 4.0 seconds, your page is considered “poor” and will be penalized in search rankings.
The most common causes of poor LCP are slow server response times, render-blocking CSS and JavaScript, and unoptimized images. Page caching addresses the server response time directly: a cached page is served as a static file, eliminating the PHP and database overhead that adds hundreds of milliseconds (or more) to every request.
Interaction to Next Paint (INP)
INP replaced First Input Delay (FID) as a Core Web Vital in March 2024. Unlike FID, which only measured the delay before the first interaction, INP measures the responsiveness of the page throughout its entire lifetime — every click, tap, and keypress. Google wants INP to be under 200 milliseconds.
Heavy JavaScript is the primary culprit behind poor INP scores. Every script that runs on your page — analytics, chat widgets, ad networks, third-party embeds — competes for the main thread. Deferring non-critical JavaScript and removing unused scripts are the most effective fixes.
Cumulative Layout Shift (CLS)
CLS measures visual stability — how much the page layout shifts as it loads. You have experienced this: you go to tap a button, and at the last moment an ad loads above it and you tap the wrong thing. Google wants CLS to be under 0.1.
The fix for CLS is straightforward: always include width and height attributes on images and iframes, reserve space for embedded content, and avoid injecting content above existing content after the page has loaded. Caching Out can add missing dimension attributes to your images automatically.
Core Web Vitals are not a pass/fail test. They are a spectrum. Every millisecond you shave off your LCP or INP is a measurable improvement in user experience — and in search visibility.
Further reading: How Page Caching Works (and Why Your Site Needs It) for a deep dive into the optimization that has the biggest single impact on LCP.