Understanding Core Web Vitals: A Guide to Website Performance Metrics

The Core Web Vitals are a set of three website performance metrics that capture how users experience the performance of your website. They are used as a search result ranking factor by Google. Site owners who optimize the performance of their website will rank higher in Google search results.

What Are Core Web Vitals?

Google has defined thresholds for each of these metrics to rate them as "Good", "Needs Improvement", or "Poor". When improving Core Web Vitals, you should see a gradual ranking boost as your metrics move toward the "Good" rating. Once all metrics are in the "Good" range, further improvements will no longer yield SEO benefits.

How Google Collects Data

Google collects metrics from real users on your website as part of the Chrome User Experience Report (CrUX). Data is collected from:

• Chrome users on desktop and Android
• Users who have opted into usage statistics reporting
• Users who are logged into their Google account

Measuring Core Web Vitals

Core Web Vitals can be measured in two ways:

Lab Data

Lab data is determined by loading a web page in a controlled environment with a predefined set of network and device conditions.

Tools: Lighthouse, PageSpeed Insights, WebPageTest, Performance tab in Google Chrome, WebVitals JS library.

Field Data (RUM - Real User Monitoring)

Data collected from the real users visiting your site, across a variety of devices and network conditions. This is a closer indicator of real-world experience, and is taken into account by Google for rankings.

Tools: PageSpeed Insights, WebPageTest, Google Search Console, Chrome User Experience Report (CrUX), DebugBear.

The Three Core Web Vitals Metrics

As of 2024, the three Core Web Vitals metrics are:

• Largest Contentful Paint (LCP): Measures page load time
• Cumulative Layout Shift (CLS): Measures visual stability
• Interaction to Next Paint (INP): Measures responsiveness

Key Terminology

Understanding these terms will help you better optimize your website's performance:

Above the fold: The content is the part of a web page shown before scrolling. Any content you'd need to scroll down to see would be considered "below the fold".

Critical resource (CSS, JS): Any resource that is required in above-the-fold content is called a critical resource.

Long task: Any piece of code that blocks the main thread for 50 ms or more can be characterized as a Long Task.

Main thread: The main thread is where all JS tasks are run in the browser. The main thread can only process one task at a time. When tasks stretch beyond a certain point, 50 milliseconds to be exact, they're classified as long tasks. If the user is attempting to interact with the page while a long task runs or if an important rendering update needs to happen, the browser will be delayed in handling that work. This results in interaction or rendering latency.

Conclusion

Optimizing Core Web Vitals is essential for both user experience and SEO. By understanding these metrics and using the right tools to measure them, you can improve your website's performance and search rankings.

Hope this guide helps you understand Core Web Vitals! If you have any questions, feel free to reach out to me on LinkedIn.