For web developers

Core Web Vitals: what to fix first

A beautiful site that feels slow still loses people. Three numbers tell you where to start.

By SlapUp · 4 November 2026 · 4 min read

Core Web Vitals measure how a page feels to real visitors: how quickly the main content appears, how quickly the page responds and how stable it stays. A page is judged on the 75th percentile of its page views, so a fast result on your own laptop doesn't settle it.

Start with real data

Run your page through PageSpeed Insights and look for the field data, which comes from real visitors, if your site has enough traffic. Lab scores are useful for debugging, but they are a simulation.

LCP: make the main content arrive sooner

• Compress your hero image and serve a modern format, such as WebP or AVIF, at the right size.
• Don't lazy-load the main image, and consider giving it a high fetch priority.
• Reduce render-blocking CSS and scripts, and improve slow server responses.

INP: keep the page responsive

• Break long JavaScript tasks into smaller pieces.
• Remove or delay third-party scripts you don't truly need.
• Show a visual response right away, then do the heavy work afterwards.

CLS: stop things jumping

• Set width and height, or an aspect ratio, on images and video.
• Reserve space for banners and embeds before they load.
• Use sensible font loading, so text doesn't jump when the font arrives.

What about impressive animation?

You don't have to choose between a striking site and a fast one, but you do need a budget. Load heavy effects after the main content, use fewer of them on phones, respect reduced-motion settings and pause them when they're off screen.

The short version

Fix the slowest real page first, usually starting with its main image and its scripts. Then measure again.