Lighthouse Score Simulator

This tool provides estimates based on simplified heuristics — not actual browser measurement. Real Lighthouse scores depend on many additional factors including server response headers, JavaScript execution time, third-party scripts, layout shifts, font loading, and network conditions. Use this tool to understand which resource types impact performance most, not as a substitute for running Lighthouse in Chrome DevTools or CI.

LCP (Largest Contentful Paint) measures how long it takes for the largest visible content element — usually a hero image or heading — to render. Google's Core Web Vitals threshold is LCP ≤ 2.5s (Good). LCP is the most important single metric for perceived load performance and is a direct Google Search ranking signal since 2021.

Render-blocking resources are scripts and stylesheets that prevent the browser from rendering any content until they are fully loaded and parsed. A <script> tag in <head> without async or defer blocks rendering. A <link rel=stylesheet> blocks rendering until the CSS is parsed. To eliminate render-blocking: add async/defer to scripts, inline critical CSS, and lazy-load non-critical styles.

Lighthouse categories scores as: 90-100 = Good (green), 50-89 = Needs Improvement (orange), 0-49 = Poor (red). For competitive web pages, aim for 90+ on desktop and 70+ on mobile. Mobile scores are typically lower due to CPU throttling (4x) and network throttling (Fast 3G) that Lighthouse applies in mobile mode.

It depends on your bottleneck. For most sites: (1) Compress and lazy-load images (use WebP/AVIF, add loading=lazy). (2) Code-split your JavaScript — large bundles are the #1 killer of Time to Interactive. (3) Eliminate render-blocking resources with async/defer. (4) Improve TTFB by using a CDN and caching static assets. Run Lighthouse to see your specific Opportunities and Diagnostics.