Core Web Vitals explained and optimized

What are Core Web Vitals?

Core Web Vitals are three metrics that Google defines as crucial to a good user experience:

• Largest Contentful Paint (LCP)
  Measures how fast the largest visible element (usually an image or headline) is rendered. Target: within 2.5 seconds.
• Cumulative Layout Shift (CLS)
  Measures how much content shifts unexpectedly during loading. Target score: under 0.1.
• Interaction to Next Paint (INP)
  Replaced the old FID metric in March 2024. Measures how fast a page responds to interactions like clicks or taps. Target: under 200 ms.

All three are based on field data, meaning real-world user experience rather than synthetic tests.

Why they matter

A slow or unstable site drives users away. That may sound dramatic, but it's increasingly backed by data:

• Improving LCP by one second can increase conversions by up to 15% (source: Google).
• High CLS scores lead to frustration and lower satisfaction.
• Slow interaction (high INP) causes drop-offs, especially on mobile.

Google also pays attention. Core Web Vitals are a ranking factor in search results. They may not dominate the algorithm, but they make a difference when your competitor is equally relevant.

In short: optimizing for Core Web Vitals isn't just technical polish. It's a direct investment in user experience and visibility.

How to measure your scores

There are several ways to measure Core Web Vitals. Most are free and easy to use:

• PageSpeed Insights
  Shows both lab data and field data per page. Includes a breakdown by metric.
• Lighthouse (Chrome DevTools)
  Great for local audits during development. Combines performance, SEO and accessibility checks.
• Web Vitals Chrome Extension
  Real-time feedback while browsing your site. Useful for continuous monitoring.
• Search Console > Page Experience
  Highlights poorly performing URLs based on real user data from Chrome.

Note: scores vary per page, device, and time. Use multiple tools and focus on trends, not just snapshots.

How to improve them, metric by metric

Improving LCP

• Optimize images
  Use modern formats like WebP or AVIF. Do not lazy-load elements in the viewport.
• Reduce server response time (TTFB)
  Use caching, edge delivery, or a faster backend.
• Minimize render-blocking resources
  Defer or reduce CSS and JavaScript that delay first render.

Improving CLS

• Set fixed dimensions for elements
  Especially for images, iframes, and ads. Prevent layout shifts.
• Use font-display: swap
  Avoid flashes of invisible text (FOIT).
• Load UI components predictably
  Do not inject dynamic elements without reserving space.

Improving INP

• Optimize JavaScript execution
  Split large bundles, use code splitting, minimize third-party scripts.
• Prioritize interactive elements
  Ensure buttons and forms respond instantly, even under load.
• Use a performant framework or improve your own
  Next.js, SvelteKit, Astro. All help enable instant interactivity.

Common pitfalls

Many teams chase scores but forget what they reflect: actual experience.

• A high Lighthouse score means little if mobile users still wait four seconds.
• Lazy loading might improve LCP but worsen CLS. Tradeoffs matter.
• Over-optimizing for lab data can degrade real-world performance.

Trust your tools, but always test with real users. Combine metrics with common sense.

How we do it at Forge

At Forge, we start with real-user data. No optimization without context.

We collaborate with teams to embed performance into the design and development process from day one. Think:

• Component libraries that prevent layout shifts
• Early performance audits during prototyping
• Hosting strategies that structurally reduce LCP and INP

In summary

Core Web Vitals are not just technical KPIs. They measure what users feel—speed, stability, responsiveness. By understanding and improving them, you're not just building faster sites. You're delivering better experiences.

Curious how your site performs? We are happy to take a look. A performance audit often reveals quick wins right away.