🔦Understanding Lighthouse Scores
How Google Lighthouse calculates performance scores, what the thresholds mean, and how to interpret the scoring curve.
Google Lighthouse is an open-source tool that audits web pages for performance, accessibility, best practices, and SEO.[1] Hugo uses the Lighthouse engine (via PageSpeed Insights API) to generate performance scores during Premium analyses.
How Scores Are Calculated
The Lighthouse performance score is a weighted average of five metrics:[2]
| Metric | Weight | What It Measures |
|---|---|---|
| Total Blocking Time (TBT) | 30% | Main thread responsiveness |
| Largest Contentful Paint (LCP) | 25% | Loading — when largest element appears |
| Cumulative Layout Shift (CLS) | 25% | Visual stability during load |
| First Contentful Paint (FCP) | 10% | When first content appears |
| Speed Index | 10% | How quickly content is visually complete |
Scoring Curve
Lighthouse uses a log-normal distribution to convert raw metric values into 0–100 scores.[2] This means improvements at the lower end of the scale have a larger impact than at the higher end — going from 20 to 50 is easier than going from 80 to 90.
Lighthouse scores can vary between runs due to network conditions, server load, and background processes. Google recommends averaging multiple runs for reliable benchmarking.[2]
Score Ranges
| Range | Color | Interpretation |
|---|---|---|
| 90–100 | 🟢 Green | Good — fast, responsive, stable |
| 50–89 | 🟠 Orange | Needs Improvement — visible performance issues |
| 0–49 | 🔴 Red | Poor — significant performance problems |
Common Misconceptions
- A score of 100 doesn't mean your page is perfectly fast — it means it's in the top percentile of web performance.[2]
- Mobile and desktop scores differ significantly — Google scores mobile performance on throttled conditions.[2]
- Lighthouse measures lab data (simulated), not field data (real users). Both matter for SEO.[3]
- Third-party scripts (ads, analytics, chat widgets) can heavily impact your score but may be unavoidable.
References
- [1]Google Developers — Lighthouse overview — developer.chrome.com
- [2]Chrome Developers — Lighthouse performance scoring — developer.chrome.com
- [3]web.dev — How metrics are measured (Lab vs Field) — web.dev