Understanding Web Vitals and Core Web Vitals

A brief explanation of Web Vitals and Core Web Vitals.

2025/03/25

What Are Web Vitals?

Web Vitals are a unified set of metrics introduced by Google to measure the quality of user experience on web pages. These metrics quantify real-world user experience and help developers identify and improve site performance issues. Among these, the most important metrics are known as Core Web Vitals.

Note that Web Vitals may change over time, and any updates are reflected in the following change log.

https://chromium.googlesource.com/chromium/src/+/master/docs/speed/metrics_changelog/README.md

Core Web Vitals

Currently, Core Web Vitals consist of the following three metrics:

Each metric is evaluated based on the 75th percentile of page loads, segmented across mobile and desktop devices.

1. LCP (Largest Contentful Paint) – Loading Performance

What it measures: The time it takes for the largest content element visible in the viewport to load.
Ideal value: Less than or equal to 2.5 seconds.

LCP measures how quickly the main content of a page becomes visible to users. It reflects the perceived loading speed.

2. INP (Interaction to Next Paint) – Interactivity

What it measures: The latency between user interaction (click, tap, key press, etc.) and the next visual response.
Ideal value: Less than 200 milliseconds.

INP is a newer metric replacing First Input Delay (FID). It assesses the overall responsiveness of a page by capturing the longest interaction latency during a user’s visit. It indicates how smooth and responsive the site feels to users.

3. CLS (Cumulative Layout Shift) – Visual Stability

What it measures: The amount of unexpected layout shifts during the page’s lifecycle.
Ideal value: Less than 0.1.

CLS quantifies visual instability—when content suddenly moves on screen. This could be caused by things like ads loading late and pushing content down or buttons shifting position, leading to accidental clicks.

Other Key Web Performance Metrics (Non-Core Web Vitals)

In addition to Core Web Vitals, the following metrics are also used as supporting indicators of performance. However, unlike Core Web Vitals, they may not always directly reflect real user experience and should be considered supplementary.

FCP (First Contentful Paint) – Loading Performance

What it measures: Time until the browser renders the first piece of content from the DOM.
Ideal value: Less than or equal to 1.8 seconds.

TTFB (Time to First Byte) – Loading Performance

What it measures: Time until the browser receives the first byte of data from the server.
Ideal value: Less than 0.8 seconds.

TBT (Total Blocking Time) – Interactivity

What it measures: The total time during which the main thread was blocked and unable to respond to user input, occurring after FCP.
Ideal value: Less than 200 milliseconds. TBT is often used as an alternative mertics for INP.

Conclusion

Web Vitals are more than just technical metrics—they reflect real user experience. Improving the three Core Web Vitals (LCP, INP, and CLS) is not only essential for a better user experience, but also directly impacts search engine rankings (SEO), and by extension, business performance.

Regularly measuring and continuously improving these metrics will help you build a more user-friendly website. Performance optimization is not a one-time task but an ongoing process. Use Web Vitals and Core Web Vitals to enhance user satisfaction and drive business value.