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Core Web Vitals What are Core Web Vitals?

The Core Web Vitals are a set of three metrics defined by Google to measure the user experience of a web page. These metrics are an official ranking factor for SEO since 2021.

To pass the Core Web Vitals assessment, all three metrics must meet the "good" threshold at the 75th percentile on field data (real user data collected by Chrome).

LCP Loading - ≤ 2.5s INP Interactivity - ≤ 200ms CLS Stability - ≤ 0.1

Largest Contentful Paint (LCP)

Loading time of largest visible element

Thresholds: ≤ 2.5s good | > 4s bad

What is the LCP?

LCP measures the time it takes to display the largest visible content element in the viewport. This typically includes hero images, videos, or large blocks of text.

How to improve the LCP

  • Optimize images: Use modern formats (WebP, AVIF), compress them and set explicit dimensions
  • Preload critical resources: Use <link rel="preload"> for LCP images and fonts
  • Improve server response time: Use a CDN, optimize the backend, cache responses
  • Eliminate blocking CSS: Inline critical CSS and defer the rest
  • Avoid layout changes: Always set width and height on images

Interaction to Next Paint (INP)

Responsiveness to user interactions

Thresholds: ≤ 200ms good | > 500ms bad

What is the INP?

The INP measures the latency of all user interactions (clicks, presses, keyboard entries) during the entire duration of a page visit. A good INP means that the page responds quickly to user actions.

How to improve INP

  • Reduce JavaScript: Break long JavaScript tasks (>50 ms) into smaller tasks
  • Defer non-critical JavaScript: Use defer or async on scripts
  • Avoid heavy calculations: Use Web Workers for Costly Operations
  • Optimize event handlers: Use event delegation and debounce/throttle
  • Minify the DOM: Smaller DOM = faster rendering updates

Cumulative Layout Shift (CLS)

Visual stability of the page

Thresholds: ≤ 0.1 good | > 0.25 bad

What is CLS?

CLS measures visual stability by quantifying unexpected layout shifts. A bad CLS frustrates users who may click on the wrong element due to a sudden move.

How to improve CLS

  • Reserve space for images/videos: Always set attributes width and height
  • Avoid dynamic ads: Reserve a fixed space for advertising locations
  • Preload fonts: Use font-display: optional or preload critical fonts
  • Avoid inserting content on top of existing content: Except in response to user interaction
  • Use transform animations: Prefer transform à top/left for the entertainment
Indirect factors

The following metrics do not not part of Core Web Vitals and do not directly influence the Pass/Fail evaluation. However, they can be the underlying cause of a bad LCP or INP. Optimizing these metrics can indirectly improve your Core Web Vitals.

First Contentful Paint (FCP)

First view of content

Indirect
Thresholds: ≤ 1.8s good | > 3s bad

What is the FCP?

The FCP measures the time between the start of navigation and the moment when the browser displays the first element of content (text, image, canvas). A bad FCP indicates that the server or initial renderer is too slow, causing also delays the LCP.

Impact on Core Web Vitals

The FCP is a precursor to the LCP: if the first content takes time to display, the largest element will necessarily take longer too. Improving the FCP generally improves the LCP.

How to improve FCP

  • Reduce TTFB: A slow server delays everything else
  • Remove blocking resources: CSS and JS in the <head> block rendering
  • Inline critical CSS: The CSS necessary for the first display must be inline
  • Preconnect to third-party origins: <link rel="preconnect"> for CDN, fonts, etc.
  • Minimize CSS/JS: Reduce the size of files to download

Time to First Byte (TTFB)

Server response time

Indirect
Thresholds: ≤ 800ms good | > 1.8s bad

What is TTFB?

TTFB measures the time between the HTTP request and receipt of the first byte of the response. This is the time it takes for your server to “respond”. A high TTFB delay everything else : FCP, LCP, and even interactivity.

Impact on Core Web Vitals

TTFB is the foundation for all other metrics. If your server responds slowly, all your performance metrics will be impacted. It is often the first thing to optimize.

How to improve TTFB

  • Use a CDN: Serve content from servers close to users
  • Cache pages: Server cache (Redis, Varnish) or application cache
  • Optimize database queries: Indexes, N+1 queries, pagination
  • Use HTTP/2 or HTTP/3: Faster protocols
  • Avoid redirects: Each redirect adds a network round trip
  • Optimize the backend: Efficient server code, good hosting

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