# Speed Index

Speed Index measures how quickly the visible part of a page fills with content, computed from a filmstrip of the loading page. A lower number is better. It is a lab metric from Lighthouse and WebPageTest, not a field metric, and not a Core Web Vital.

Updated 2026-09-15 · Source: https://porteur.ai/glossary/speed-index

## What it measures that other metrics do not

Most timings mark one moment: the first paint, the largest paint, the first interaction. Speed Index scores the whole fill: a page that paints a header at half a second and the rest at four seconds scores worse than one that paints everything at two.

| Metric | What it marks |
| --- | --- |
| First Contentful Paint | The first pixel of content |
| Largest Contentful Paint | The largest element in the viewport |
| Speed Index | The average progress of the whole visible area |
| Total Blocking Time | How long the main thread was blocked |

## Where it appears and what it weighs

- Lighthouse, in Chrome DevTools, in PageSpeed Insights and in the command line tool.
- WebPageTest, which produces the filmstrip the metric is computed from.
- It carries 10% of the Lighthouse performance score in version 10 and later.
- It is not in the Chrome User Experience Report, so it never appears as field data in Search Console.

> A lab metric is one simulated load on one simulated device. The figure Google ranks on is the field data of real visitors, which holds the three Core Web Vitals and not this one.

## When it is worth looking at

1. **When the page paints in stages** A high Speed Index with a good First Contentful Paint means something paints early and the rest arrives late. Look at what fills the viewport last.
2. **When an image or a font is late** A hero image discovered late, or text that waits for a web font, both show up here before they show up anywhere else.
3. **When comparing two builds** Run it before and after a change on the same page and the same settings. It is a good relative measure and a poor absolute one.

When the Core Web Vitals are already good in the field, Speed Index is a curiosity. When they are not, it helps explain which part of the load is slow.

## Questions

### What is a good Speed Index?

Lighthouse scores it against its own scale and it varies with the device and connection simulated. Treat it as a relative measure between two runs of the same page rather than a target.

### Speed Index or Largest Contentful Paint?

Largest Contentful Paint decides the field assessment and therefore matters more. Speed Index explains a page that fills in stages, which LCP alone does not show.

### Is Speed Index a ranking factor?

No. It is a lab metric and is not part of the Core Web Vitals that Google uses, which are LCP, CLS and INP measured on real visitors.

### Why does Speed Index change between runs?

It is a simulated load: network, CPU and caching vary each time. Run it a few times and compare medians, on the same settings.

### Where do I see Speed Index?

In the Lighthouse section of PageSpeed Insights, in Chrome DevTools, or in WebPageTest, which shows the filmstrip the number comes from.

## Read next

- [Core Web Vitals](https://porteur.ai/glossary/core-web-vitals): Core Web Vitals are Google’s three field speed and responsiveness metrics. See their thresholds, lab versus field, how to read them, and what to fix.
- [Largest Contentful Paint (LCP)](https://porteur.ai/glossary/largest-contentful-paint): Largest Contentful Paint is when the biggest element appears. Aim under 2.5 s. See how to measure it, usual causes on small sites, and quick fixes.
- [How to read PageSpeed Insights: field data first, then the score](https://porteur.ai/guides/how-to-read-pagespeed-insights): Start with field data, not the score. Learn what Google uses, why scores vary, and how to turn a PageSpeed Insights run into a clear order of fixes.
- [Reading a Lighthouse score on a phone](https://porteur.ai/guides/lighthouse-score): Use the mobile Lighthouse run to judge speed and UX. See what each score means, how lab and field data differ, and what to fix first.
- [First Contentful Paint: what it measures and what moves it](https://porteur.ai/guides/first-contentful-paint): What First Contentful Paint measures, the good and poor thresholds, why it affects Lighthouse not rankings, and how to make it faster on your site.
- [Total Blocking Time: the lab number that stands in for INP](https://porteur.ai/guides/total-blocking-time): What Total Blocking Time measures in Lighthouse, why it weighs 30 percent, how it relates to INP, and the fixes that cut it fast.
- [Core Web Vitals checker](https://porteur.ai/tools/core-web-vitals-checker): Paste a URL for one Lighthouse run on a simulated phone: LCP, CLS, TBT and FCP against Google's thresholds, the biggest savings, the failing audits.

The free check runs a measurement of your home page on a simulated phone and names the change that would save the most time, in about thirty seconds. Free check: https://porteur.ai/