Performance metric
What is Speed Index?
Speed Index is a Lighthouse lab metric that estimates how quickly visible content appears during load. Lower is better because the viewport fills in sooner.
Plain-English version
How quickly visible content fills the screen.
Target
Under 3.4s
SI thresholds
Use thresholds as a triage tool. Field data matters most when there is enough real Chrome traffic for the page.
Good
≤ 3.4s
Needs work
3.4–5.8s
Poor
> 5.8s
Why it matters
What a visitor feels when SI is bad.
Why SI matters
Speed Index is useful when a page does not stay blank but still feels like it fills in slowly. It can reveal a poor visual loading experience that one metric alone misses.
What a poor result usually means
A poor Speed Index often means render-blocking resources, slow images, too much JavaScript, or late-loading above-the-fold content.
What to fix first
Do not chase the score. Fix the bottleneck.
The right fix depends on the metric, the page template, and whether the issue appears in real visitor data.
- Improve the first render path for above-the-fold content.
- Prioritize important images and defer below-the-fold assets.
- Reduce render-blocking CSS and early JavaScript.
- Avoid large empty shells that wait for client-side data.
- Compare Speed Index with LCP and FCP before deciding what to fix.
How nimo helps
Ask nimo: “How can I improve my Speed Index score?”
nimo uses Speed Index as supporting evidence, then ties it back to the visitor-facing metrics that matter most.
No signup needed. Takes about 30 seconds.