SI
Speed Index
How quickly the visible page content is visually populated — lab metric only.
- Good: ≤ 1.3s
- Needs improvement: ≤ 2.3s
- Poor: > 2.3s
0.0 s
0.5 s
1.0 s
1.5 s
2.0 s
Visually complete0256085100%
Frames captured while the page loadsFrames captured while the page loadsFrames captured while the page loadsFrames captured while the page loadsSpeed Index: how fast the screen fills
What is Speed Index?
Speed Index (SI) measures how quickly the visible portion of a page is visually populated during load. It is computed from a video recording of the page load by measuring the average time at which visible parts of the page are displayed.
What problem does it solve?
Unlike FCP which fires on the first pixel, Speed Index captures the entire visual progression — a page that loads content incrementally scores better than one that paints everything at once after a long delay.
What is a good Speed Index score?
s
Good
Exceptional — page fills very rapidly.
How to improve Speed Index
Speed Index improvements are downstream of fixing TTFB, FCP, and above-the-fold loading.
01
Server response and rendering foundation
Speed Index cannot improve beyond the rate at which the browser receives HTML and can begin rendering.
Fix Page caching, CDN, and critical CSS inlining all contribute.
02
Above-the-fold resource loading
Images and fonts visible in the initial viewport that load slowly delay visual completeness.
Fix Lazy-load images below the fold only. Preload above-the-fold images. Subset and preload critical web fonts.
03
JavaScript execution during load
JavaScript that runs during page load and manipulates the DOM can delay rendering.
Fix Prefer server-side rendering for above-the-fold content. Defer non-critical JavaScript.
Need help fixing your SI?
Root cause analysis, not plugin guessing. Baseline from field data or lab — wherever you are.
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