Core Web Vitals: how to fix LCP, INP and CLS

Core Web Vitals are Google's user-experience metrics for loading, responsiveness and visual stability. They are a ranking signal among many — rarely decisive alone, but slow pages also convert worse and frustrate visitors coming from AI answers.

The three metrics

INP replaced First Input Delay (FID) as a Core Web Vital in March 2024.

Fixing LCP

1. Speed up the server. Page caching, a CDN, and fewer slow database queries cut Time to First Byte.
2. Optimise the LCP image. Correct dimensions, WebP or AVIF, fetchpriority="high" and never loading="lazy" on it.
3. Remove render-blocking resources. Defer non-critical JavaScript, inline critical CSS, preload the main font.

Fixing INP

1. Remove unused JavaScript and heavy third-party tags (chat widgets, multiple analytics, A/B testing).
2. Break long tasks into smaller chunks and avoid big work in event handlers.
3. Keep the DOM small; huge pages slow every interaction.

Fixing CLS

1. Give images, videos and iframes width and height (or CSS aspect-ratio).
2. Reserve space for ads, embeds and cookie banners.
3. Avoid inserting content above existing content; use font-display: swap with similar fallback fonts.

Lab vs field data

PageSpeed Insights shows both: the lab test (one simulated visit) for diagnosis and, if your site has enough traffic, field data from real Chrome users over 28 days. Google uses field data. After fixing, expect up to four weeks before the field numbers fully update.

BoostSEO runs PageSpeed Insights on your start page and lists the biggest opportunities in the Performance tab.