Site Speed & Core Web Vitals 101

What are Core Web Vitals?

Core Web Vitals are a set of metrics that Google considers important in a web page’s overall user experience. These metrics are focused on three core elements of a user’s experience:

• Website load time
• Interactivity
• Visual stability

These elements are measured using the following metrics:

• Largest Contentful Paint (LCP): A measurement of the time it takes for a page’s content to load.
• First Input Delay (FID): A measurement of the time it takes for a user’s action on a page to process. For example, clicking a button or submitting a form.
• Cumulative Layout Shift (CLS): This metric assesses visual stability of a page’s layout, measuring how often users experience unexpected layout shifts. For example, when you’re scrolling through an article and the page jumps due to ads or graphics.

Tools used to measure site speed and Core Web Vitals:

• PageSpeed Insights
• Lighthouse
• GTMetrics – non-Google tool, but still uses Lighthouse data

Core Web Vitals Glossary

• Core Web Vitals: Central metrics for web performance, part of the broader Web Vitals initiative by Google.
• PageSpeed Insights: A Google tool that uses field data and also lab data from Lighthouse to measure performance. This includes Core Web Vitals metrics.
• Lighthouse: An open-source tool that provides comprehensive performance audits, including Core Web Vitals, and powers the performance analysis in PageSpeed Insights.
• GTmetrix: A performance analysis tool that uses proprietary field data and Lighthouse, includes Core Web Vitals, and provides additional proprietary metrics and insights.

PageSpeed Insights 

PageSpeed Insights (PSI) is a tool provided by Google that analyzes the content of a web page, then generates suggestions to make that page faster. PSI provides both lab and field data about a page. Lab data is useful for debugging performance issues, as it is collected in a controlled environment. However, it may not capture real-world bottlenecks. Field data is useful for capturing true, real-world user experience – but it may not identify all the potential performance issues.

Relationship with Core Web Vitals: PSI includes Core Web Vitals metrics in its reports, giving insights into how well a page performs in terms of LCP, FID, and CLS.

Lighthouse

Lighthouse is an open-source, automated tool for improving the quality of web pages. It provides audits for performance, accessibility, progressive web apps, SEO, and more. You can run Lighthouse in Chrome DevTools, from the command line, or as a Node module.

Relationship with Core Web Vitals and PageSpeed Insights: Lighthouse powers the performance audits in PageSpeed Insights. It also includes Core Web Vitals metrics as part of its performance audits, providing deeper insights into specific areas of web page performance.

GTmetrix

GTmetrix is a non-Google owned tool that provides insights into your page performance, offering detailed reports on various performance metrics and providing actionable recommendations to improve your site’s speed. It uses Lighthouse and other performance analysis tools to generate its reports.

Relationship with Core Web Vitals: GTmetrix includes Core Web Vitals metrics in its performance reports, offering a comprehensive view of a site’s loading, interactivity, and visual stability.

Relationship with Lighthouse: GTmetrix leverages Lighthouse to provide detailed performance analysis, along with other proprietary metrics and data to give a holistic view of site performance.