What Are Core Web Vitals?

Core Web Vitals are a set of specific factors that Google considers important for a webpage’s overall user experience. These metrics focus on three aspects of user experience: loading performance, interactivity, and visual stability.

The Three Core Metrics

1. Largest Contentful Paint (LCP)

LCP measures loading performance. To provide a good user experience, LCP should occur within 2.5 seconds of when the page first starts loading.

What it measures:

  • The render time of the largest content element visible in the viewport
  • Could be an image, video, or large text block

How to optimize:

  • Optimize and compress images
  • Implement lazy loading
  • Use a Content Delivery Network (CDN)
  • Minimize render-blocking resources

2. First Input Delay (FID)

FID measures interactivity. Pages should have an FID of less than 100 milliseconds to provide a good user experience.

What it measures:

  • The time from when a user first interacts with your page to when the browser responds
  • Examples: clicking a button, tapping a link, or using a custom JavaScript control

How to optimize:

  • Break up long JavaScript tasks
  • Optimize your page for interaction readiness
  • Use a web worker
  • Reduce JavaScript execution time

3. Cumulative Layout Shift (CLS)

CLS measures visual stability. Pages should maintain a CLS of less than 0.1 to ensure a good user experience.

What it measures:

  • The sum of all unexpected layout shifts that occur during the page’s lifespan
  • Common causes: images without dimensions, ads, embeds, and dynamically injected content

How to optimize:

  • Always include size attributes on images and video elements
  • Never insert content above existing content
  • Use transform animations instead of animations that trigger layout changes
  • Preload fonts to avoid font swapping

Why Core Web Vitals Matter

1. User Experience

Poor performance leads to frustrated users who abandon your site. Studies show:

  • 53% of mobile users leave a site that takes longer than 3 seconds to load
  • Users are 70% more likely to convert on fast-loading sites
  • Every 100ms delay in load time can hurt conversion rates by 7%

2. SEO Rankings

Google now uses Core Web Vitals as part of its ranking algorithm. Better performance can lead to:

  • Higher search rankings
  • Increased organic traffic
  • Better visibility in search results

3. Business Impact

Performance directly impacts your bottom line:

Faster Load Times = Better User Experience = Higher Conversions

Measuring Core Web Vitals

Tools to Use

  1. Google PageSpeed Insights - Comprehensive analysis with recommendations
  2. Chrome DevTools - Real-time performance monitoring
  3. Lighthouse - Automated auditing for performance, accessibility, and more
  4. Web Vitals Extension - Real-time Core Web Vitals metrics in your browser

Monitoring in Production

// Example: Track Core Web Vitals with the web-vitals library
import {getCLS, getFID, getLCP} from 'web-vitals';

function sendToAnalytics(metric) {
  // Send to your analytics endpoint
  console.log(metric);
}

getCLS(sendToAnalytics);
getFID(sendToAnalytics);
getLCP(sendToAnalytics);

Optimization Strategies

Image Optimization

  • Use modern formats like WebP or AVIF
  • Implement responsive images with srcset
  • Lazy load images below the fold
  • Compress images without losing quality

JavaScript Optimization

  • Code splitting for larger applications
  • Tree shaking to remove unused code
  • Minification and compression
  • Defer non-critical JavaScript

CSS Optimization

  • Minimize critical CSS
  • Remove unused CSS
  • Use CSS containment
  • Optimize CSS delivery

Real-World Example

Let’s look at how a simple change can improve LCP:

Before:

<img src="hero-image.jpg" alt="Hero" />

After:

<img 
  src="hero-image.webp" 
  alt="Hero"
  width="1200"
  height="600"
  loading="eager"
  fetchpriority="high"
/>

This change:

  • Uses a modern, compressed format (WebP)
  • Prevents layout shift with explicit dimensions
  • Prioritizes loading of the hero image

Conclusion

Core Web Vitals aren’t just metrics—they’re a reflection of your users’ experience. By focusing on these key performance indicators, you’re not just improving numbers; you’re creating faster, more enjoyable websites that users love and search engines reward.

Start measuring your Core Web Vitals today and make performance a priority in your development workflow. Your users (and your business) will thank you!