TL;DR — Quick Summary
Speed Index measures how quickly the visible area of the page is populated with content. Good Speed Index is ≤ 3.4 seconds (Lighthouse). It captures the overall visual loading experience — rewarding progressive rendering over delayed all-at-once rendering. Lab-only metric, 10% of Lighthouse score.
What is Speed Index (SI)?
Speed Index measures how quickly content is visually displayed during page load by taking video frames and computing the average time at which visible parts appear. Lower is better.
The calculation works as follows:
- 1Record the visual loading process as video frames (typically 10 frames/second).
- 2For each frame, calculate the percentage of visual completeness compared to the final rendered state.
- 3Compute the area above the visual progress curve.
A page that renders 80% of content at 1 second and finishes at 2 seconds has a much better Speed Index than a page that shows nothing until 1.8 seconds and finishes at 2 seconds — even though their LCP may be identical.
Speed Index is a lab-only metric — it requires video capture of the loading process, which is only available in controlled environments like Lighthouse and WebPageTest.
SI Thresholds
| Metric | Good | Needs Improvement | Poor |
|---|---|---|---|
| Speed Index | ≤ 3.4s | 3.4s – 5.8s | > 5.8s |
Google evaluates the 75th percentile (p75) of real-user field data over a rolling 28-day window.
History & Evolution
Key milestones:
- •2012 — Speed Index introduced by WebPageTest creator Pat Meenan as a way to quantify the visual loading experience beyond single-point metrics.
- •2016 — Lighthouse adopts Speed Index as a core performance metric.
- •2020 — Core Web Vitals announced. Speed Index is not a CWV but remains in Lighthouse.
- •2025–2026 — Speed Index carries 10% of the Lighthouse performance score.
How SI is Measured
Speed Index is measured by recording visual loading as frames and computing the area above the visual progress curve.
Available in:
- •Lighthouse (10% of performance score)
- •WebPageTest (where it was invented)
- •PSI lab section
Key rule: Field data (CrUX) determines Google rankings. Lab data (Lighthouse, WebPageTest) is for debugging and iteration.
Common Causes of Poor SI Scores
- 1Render-blocking resources — CSS/JS that prevents any content from appearing.
- 2All-at-once rendering — Pages that show nothing, then render everything (SPAs without SSR).
- 3Large above-the-fold images without preloading — Hero images discovered late.
- 4Slow TTFB — Delays everything, pushing the entire visual progress curve later.
- 5Web font blocking — Font-display: block causes invisible text until fonts load.
Frequently Asked Questions
For step-by-step optimization, platform-specific fixes, code examples, and case studies, read our full guide:
The Ultimate Guide to Core Web Vitals: How to Pass All Metrics & Boost Rankings in 2026Struggling with SI?
Request a free speed audit and we'll identify exactly what's holding your scores back.