TL;DR
Speed Index (SI) is a lab metric that expresses, in seconds, how quickly the visible portion of a page is visually populated during load by scoring the progression of frames captured during a synthetic test.
What is Speed Index (SI)?
Speed Index (SI) is a lab metric that expresses, in seconds, how quickly the visible portion of a page is visually populated during load by scoring the progression of frames captured during a synthetic test.
Learn more in our how Speed Index is measured and used guide, or work with us through our done-for-you speed optimization service.
SI Thresholds & Benchmarks
| Metric | Good | Needs Improvement | Poor |
|---|---|---|---|
| Speed Index | ≤ 3.4 s | 3.4 s, 5.8 s | > 5.8 s |
How SI is Measured
Speed Index is computed by WebPageTest and Lighthouse from a filmstrip of the page load.