Glossary

What is Speed Index?

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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

Mobile

ScoreRating
≤ 3.4sGood
3.4s - 5.8sNeeds Improvement
> 5.8sPoor

Desktop

ScoreRating
≤ 1.3sGood
1.3s - 2.3sNeeds Improvement
> 2.3sPoor

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

MetricMeasures
FCPFirst content appears
LCPLargest content appears
Speed IndexOverall visual progress

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

Common Issues

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

Measure Speed Index

  • Lighthouse in Chrome DevTools
  • PageSpeed Insights
  • WebPageTest (original source of the metric)
Speed Index is a lab metric, not a Core Web Vital. Focus on LCP, CLS, and INP for search ranking impact.
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LCP

LCP measures loading performance. Learn what it is, thresholds, and how to improve your score.

TBT

TBT measures main thread blocking during load. Learn what it is, thresholds, and how to reduce blocking time.