Lighthouse Score: Complete Guide to Achieving 100/100

What is Google Lighthouse?

Google Lighthouse is an open-source, automated tool for auditing web page quality. It runs a series of tests against a page and generates a report on how well the page performed across five categories: Performance, Accessibility, Best Practices, SEO, and Progressive Web App.

Lighthouse is integrated into Chrome DevTools, available as a browser extension, and can be run from the command line or as a Node module. It powers Google PageSpeed Insights and is the engine behind many web performance tools including MakeWebsite.fast.

The Performance score (0-100) is what most developers focus on, but all categories contribute to overall site quality and user experience.

• Performance: Measures loading speed and interactivity using Core Web Vitals
• Accessibility: Tests for common accessibility issues that affect users with disabilities
• Best Practices: Checks for modern web development best practices and security issues
• SEO: Audits basic SEO requirements like meta tags, crawlability, and mobile-friendliness
• PWA: Tests Progressive Web App requirements (only if building a PWA)

Understanding Lighthouse Performance Scoring

The Lighthouse Performance score is a weighted average of six metrics. Understanding how each metric contributes to the final score helps you prioritize optimization efforts.

The weights change occasionally as web standards evolve. As of 2025, the metrics and their approximate weights are: LCP (25%), TBT (30%), CLS (25%), FCP (10%), Speed Index (10%).

Each metric is scored on a logarithmic scale, meaning improvements at the lower end (going from 20 to 40) have more impact on the final score than improvements at the higher end (going from 80 to 100).

• LCP (25%): Largest Contentful Paint - measures loading performance
• TBT (30%): Total Blocking Time - measures interactivity (correlates with INP)
• CLS (25%): Cumulative Layout Shift - measures visual stability
• FCP (10%): First Contentful Paint - measures initial render time
• Speed Index (10%): How quickly content is visually displayed

Why a Perfect 100 Score Matters (and When It Doesn't)

A perfect Lighthouse score demonstrates technical excellence and indicates your site follows best practices. It's a competitive advantage for SEO, as Core Web Vitals are ranking factors.

However, chasing a perfect 100 can lead to diminishing returns. The difference between 95 and 100 is usually imperceptible to users. Focus on passing Core Web Vitals thresholds first (LCP < 2.5s, INP < 200ms, CLS < 0.1).

Lab scores (Lighthouse) and field scores (CrUX) often differ. Google uses field data for ranking, so a perfect lab score doesn't guarantee good field performance.

Achieving Perfect Performance: Step-by-Step

Start by running Lighthouse and examining the Opportunities and Diagnostics sections. These provide specific, actionable recommendations sorted by estimated impact.

Address the highest-impact items first. Typically, image optimization, JavaScript reduction, and server response time improvements yield the biggest gains.

Test after each change to verify improvements. Sometimes optimizations conflict - fixing one issue may create another. Iterate carefully.

• Optimize and properly size images - use WebP/AVIF formats
• Eliminate render-blocking resources - inline critical CSS, defer JS
• Reduce JavaScript execution time - code split, tree shake, remove unused code
• Improve server response time - use CDN, caching, optimize backend
• Minimize main thread work - defer non-critical scripts, use web workers
• Reduce CLS - set dimensions on images, reserve space for ads
• Preload key resources - fonts, LCP image, critical scripts

Common Lighthouse Pitfalls and Solutions

Third-party scripts are the most common obstacle to a perfect score. Analytics, chat widgets, and ads all add JavaScript that blocks the main thread and increases TBT.

Lighthouse runs on a simulated mid-tier mobile device with 4G throttling. Your fast development machine doesn't reflect real user conditions. Always test with throttling enabled.

Score variability is normal. Lighthouse scores can fluctuate 5-10 points between runs due to network conditions and server response times. Run multiple tests and focus on the median score.

• Defer third-party scripts until after page load
• Use facade patterns for heavy embeds (load YouTube thumbnail, not player)
• Self-host critical third-party scripts when possible
• Run Lighthouse in incognito mode to avoid extension interference
• Test from multiple locations to identify geographic performance issues

Lighthouse Score Checklist

Use this checklist to systematically improve your Lighthouse score. Address each item in order of priority.

Monitor your progress with regular testing. Use MakeWebsite.fast for quick checks and Chrome DevTools for detailed debugging.

• Images: Compress, use modern formats, add dimensions, lazy load
• JavaScript: Code split, defer non-critical, remove unused, minify
• CSS: Inline critical, load rest async, remove unused, minify
• Fonts: Preload, use font-display: swap, subset characters
• Server: Use CDN, enable compression, optimize TTFB
• Third-party: Audit, remove unnecessary, defer, use facades
• CLS: Set image dimensions, reserve ad space, avoid DOM injection
• Caching: Set proper cache headers, use service workers