---
title: "What Is Speed Index in PageSpeed Insights & Lighthouse? · Unlighthouse"
meta:
  "og:description": "Speed Index measures how fast visible content fills the viewport. Good score: ≤3.4s mobile, ≤1.3s desktop. 10% of Lighthouse performance score. Thresholds, tools, and fixes."
  "og:title": "What Is Speed Index in PageSpeed Insights & Lighthouse? · Unlighthouse"
  description: "Speed Index measures how fast visible content fills the viewport. Good score: ≤3.4s mobile, ≤1.3s desktop. 10% of Lighthouse performance score. Thresholds, tools, and fixes."
---

**Glossary**

# **What Is Speed Index in PageSpeed Insights & Lighthouse?**

[Copy for LLMs](https://unlighthouse.dev/glossary/speed-index.md)

Speed Index measures how quickly visible content populates the viewport during load. It captures overall visual loading experience rather than a single moment, contributing 10% to your Lighthouse score.

## [Thresholds](#thresholds)

### [Mobile](#mobile)

| **Score** | **Rating** |
| --- | --- |
| ≤ 3.4s | Good |
| 3.4s - 5.8s | Needs Improvement |
| > 5.8s | Poor |

### [Desktop](#desktop)

| **Score** | **Rating** |
| --- | --- |
| ≤ 1.3s | Good |
| 1.3s - 2.3s | Needs Improvement |
| > 2.3s | Poor |

## [How It Works](#how-it-works)

Lighthouse captures video frames during load and calculates how quickly the viewport fills with content. A page showing 80% content instantly scores better than one loading evenly over time.

## [Speed Index vs Other Metrics](#speed-index-vs-other-metrics)

| **Metric** | **Measures** |
| --- | --- |
| [**FCP**](https://unlighthouse.dev/glossary/fcp) | First content appears |
| [**LCP**](https://unlighthouse.dev/glossary/lcp) | Largest content appears |
| Speed Index | Overall visual progress |

Speed Index complements FCP and LCP by measuring the experience between these points.

## [Common Issues](#common-issues)

- Render-blocking CSS/JavaScript
- Large above-the-fold images
- Web font loading delays
- Client-side rendering

## [Measure Speed Index](#measure-speed-index)

- Lighthouse in Chrome DevTools
- [**PageSpeed Insights**](https://pagespeed.web.dev/) — Speed Index appears in the "Diagnostics" section of PageSpeed Insights results
- WebPageTest (original source of the metric)
- [**Unlighthouse Bulk PageSpeed Test**](https://unlighthouse.dev/tools/bulk-pagespeed) — test Speed Index across your entire site

Speed Index is a lab metric, not a Core Web Vital. Focus on [**LCP**](https://unlighthouse.dev/glossary/lcp), [**CLS**](https://unlighthouse.dev/glossary/cls), and [**INP**](https://unlighthouse.dev/glossary/inp) for search ranking impact. Use the [**Lighthouse Score Calculator**](https://unlighthouse.dev/tools/lighthouse-score-calculator) to see how Speed Index (10% weight) affects your overall score.

[Edit this page](https://github.com/harlan-zw/unlighthouse/edit/main/docs/glossary/speed-index.md)

[Markdown For LLMs](https://unlighthouse.dev/glossary/speed-index.md)

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[**LCP** LCP measures loading performance. Learn what it is, thresholds, and how to improve your score.](https://unlighthouse.dev/glossary/lcp) [**TBT** TBT measures main thread blocking during load. Learn what it is, thresholds, and how to reduce blocking time.](https://unlighthouse.dev/glossary/tbt)

**On this page **

- [Thresholds](#thresholds)
- [How It Works](#how-it-works)
- [Speed Index vs Other Metrics](#speed-index-vs-other-metrics)
- [Common Issues](#common-issues)
- [Measure Speed Index](#measure-speed-index)