Core Web Vitals describe three parts of the page experience: how quickly the main content appears, how responsive the page is to interaction, and how visually stable it remains while loading.

The three metrics are Largest Contentful Paint (LCP), Interaction to Next Paint (INP), and Cumulative Layout Shift (CLS).

Largest Contentful Paint (LCP)

LCP measures how long it takes the largest visible content element to render. On many pages, that element is a hero image, a large heading, or a prominent content block.

Common causes of a slow LCP include:

• Slow server response time
• A large, uncompressed hero image
• Render-blocking stylesheets
• Important content rendered only after JavaScript runs
• An image discovered late by the browser

Start by identifying the LCP element. If it is an image, serve an appropriately sized modern format and make sure the browser can discover it early. If the server is slow, investigate caching and backend work first.

Interaction to Next Paint (INP)

INP reflects how quickly a page responds to user interactions throughout a visit. It considers interactions such as clicking, tapping, and typing, then observes how long the browser takes to show the next visual update.

Long JavaScript tasks are a frequent cause of poor responsiveness. Break up heavy work, reduce unnecessary client-side code, and avoid running expensive handlers for every keystroke or scroll event.

Third-party scripts deserve attention too. Analytics, chat, advertising, and experimentation tools all compete for time on the main thread.

Cumulative Layout Shift (CLS)

CLS measures unexpected visual movement. A page feels unstable when a button moves just as someone tries to tap it or when text shifts after a late-loading asset arrives.

Typical fixes include:

• Set width and height or an aspect ratio on images and video
• Reserve space for advertisements and embeds
• Avoid inserting banners above existing content
• Use font-loading strategies that minimize changes in text size
• Animate with transforms instead of properties that trigger layout

Lab data and field data are different

A synthetic test is controlled and repeatable, which makes it useful for diagnosing a page. Field data is collected from real visits and captures the variety of devices, networks, locations, and user behavior in production.

Use both when possible: lab tests help explain a problem, while field data shows how often real users experience it.

Improve the experience, not only the number

Core Web Vitals are useful because they map to things people notice. A fast LCP helps visitors understand that the page is useful. A good INP makes controls feel responsive. A low CLS keeps the interface predictable.

Use a website performance test to find a starting point, then address the largest user-facing delay before moving to smaller optimizations.