Core Web Vitals

Definition

Core Web Vitals are a set of performance metrics introduced by Google to measure the quality of user experience on a website. They focus on loading performance, interactivity, and visual stability, which are critical factors for how users perceive and interact with web pages.

The three main Core Web Vitals are Largest Contentful Paint (LCP), which measures loading speed, First Input Delay (FID), which measures responsiveness, and Cumulative Layout Shift (CLS), which measures visual stability. Google uses these metrics as part of its ranking signals, making them important for both user experience and search engine optimisation.

Advanced

At an advanced level, Core Web Vitals are evaluated through real-world usage data (field data) collected from Chrome User Experience Reports, as well as lab data from tools like Lighthouse and PageSpeed Insights. Optimising for these metrics involves technical improvements such as efficient caching, image optimisation, reducing JavaScript execution time, and implementing a content delivery network.

With Google’s switch to Interaction to Next Paint (INP) replacing FID, measurement of responsiveness is becoming more precise. Developers must track evolving guidelines to maintain compliance and performance benefits.

Why it matters

• Directly impacts search engine rankings and visibility.
• Improves customer experience by reducing frustration.
• Increases engagement, conversions, and retention.
• Provides measurable benchmarks for web performance.

Use cases

• Optimising e-commerce websites to reduce cart abandonment.
• Improving news and media sites for faster article loading.
• Enhancing SaaS platforms to create smoother user interactions.
• Benchmarking performance during redesigns or migrations.

Metrics

• Largest Contentful Paint (LCP) under 2.5 seconds.
• First Input Delay (FID) under 100 milliseconds, transitioning to INP.
• Cumulative Layout Shift (CLS) score under 0.1.
• Continuous monitoring through Google Search Console and field data.

Issues

• Poor scores can lower search engine rankings.
• Slow or unstable websites frustrate users and reduce conversions.
• Excessive scripts, unoptimized images, or poor hosting impact performance.
• Lack of monitoring leads to unnoticed performance degradation over time.

Example

An online marketplace improves its Core Web Vitals by compressing images, minimizing render-blocking scripts, and adopting a faster hosting solution. As a result, LCP improves from 3.5 seconds to 2.1 seconds, CLS drops below 0.1, and INP remains within Google’s recommended threshold. The site records a 20 percent increase in organic traffic and a noticeable rise in mobile sales.