Performance Score
The overall Lighthouse performance score (0–100), weighted across all six lab metrics. Google targets 90+ as "Good".
Largest Contentful Paint (LCP)
How long until the biggest visible element loads. Google's target is under 2.5 seconds. LCP is a Core Web Vital and a direct ranking signal.
First Contentful Paint (FCP)
How long until the first text or image appears. A fast FCP reassures visitors the page is loading. Target: under 1.8 seconds.
Total Blocking Time (TBT)
How long the main thread is blocked, preventing user interaction. High TBT usually means render-blocking JavaScript. Target: under 200 ms.
Cumulative Layout Shift (CLS)
How much elements jump around during load. Unexpected layout shifts frustrate users and harm conversions. Target: under 0.1.
Speed Index
How quickly page content becomes visually complete. A low Speed Index means visitors see useful content faster.
Accessibility Score
How well the page meets accessibility best practices — alt text, contrast ratios, ARIA labels. Also a Lighthouse category score.
SEO Score
Lighthouse's on-page SEO checks — meta description, crawlability, tap targets, and more. A complement to our dedicated Meta Tag Analyzer.
Top Opportunities
Ranked list of specific improvements — image compression, eliminating render-blocking resources, unused CSS/JS — ordered by potential time savings.
Google has used page speed as a ranking factor since 2010 for desktop and since 2018 for mobile. The 2021 Page Experience update made Core Web Vitals — LCP, FID/INP, and CLS — direct ranking signals. A page that fails Core Web Vitals can be pushed down in results even if its content is excellent.
Beyond rankings, speed drives revenue. Amazon famously found that every 100 ms of latency cost 1% in sales. Google's own research shows 53% of mobile users abandon a page that takes more than 3 seconds to load. A slow site loses traffic twice: once from lower rankings, and again from the visitors who do arrive but leave before the page finishes loading.
Oversized images
Compress images with WebP or AVIF, set explicit width/height attributes to prevent layout shift, and use lazy loading for below-the-fold images.
Render-blocking resources
Move non-critical JavaScript to the bottom of the page or add defer/async attributes. Inline critical CSS and defer the rest.
No caching headers
Set long Cache-Control max-age values for static assets (images, CSS, JS). A CDN can serve cached copies from edge nodes close to the visitor.
Large JavaScript bundles
Use code splitting to load only the JS needed for the current page. Remove unused libraries and polyfills. Tree-shake your build output.
No CDN
A Content Delivery Network reduces time-to-first-byte by serving assets from servers geographically close to the visitor. Even free tiers (Cloudflare) dramatically improve global load times.
Poor LCP element
If the LCP element is a hero image, preload it with <link rel="preload">. If it's text, ensure the web font loads fast or use font-display:swap.
High CLS
Always set explicit width and height on images and video. Reserve space for ads and embeds. Avoid inserting content above existing content after load.
What is a good PageSpeed score?
Google classifies 90–100 as Good, 50–89 as Needs Improvement, and 0–49 as Poor. For competitive niches, aim for 90+ on both mobile and desktop.
Why does mobile score lower than desktop?
PageSpeed Insights simulates a mid-range mobile device on a 4G connection, which is far more constrained than a desktop benchmark. Most sites score 10–30 points lower on mobile.
Does PageSpeed score directly affect rankings?
The raw Lighthouse score is not a ranking signal, but the underlying Core Web Vitals (LCP, INP, CLS) are. A low performance score usually means failing Core Web Vitals, which do affect rankings.
Why does my score vary between runs?
PageSpeed Insights uses real-world network conditions and lab simulations that vary slightly. Run three tests and average the results for a reliable baseline.
What are Core Web Vitals?
Core Web Vitals are three metrics Google uses as ranking signals: LCP (loading), INP (interactivity, replacing FID in 2024), and CLS (visual stability). All three must pass to earn the "Good" label.
Should I prioritise mobile or desktop?
Google uses mobile-first indexing for all sites, so mobile scores directly affect rankings. Fix mobile first, then desktop.
Is this tool free?
Our Page Speed Optimizer is free, with no sign-up required. It calls the Google PageSpeed Insights API on your behalf and does not store any data.