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.
nimo audit lens
How nimo reads SI in an audit.
The glossary explains the metric. The free audit checks the page, separates field data from lab diagnostics, and points at the first review step.
How nimo interprets it
nimo uses Speed Index as a visual progress clue when the page is not blank but still fills in slowly.
Source to trust first
Use Lighthouse filmstrip and Speed Index for lab diagnosis. Use LCP, FCP, and CrUX data for visitor impact.
Evidence to inspect
- Filmstrip frames for above-the-fold visual progress.
- Render-blocking CSS or early JavaScript.
- Late above-the-fold images or content.
- Whether LCP and FCP point to the same bottleneck.
First fix to review
Prioritize above-the-fold rendering and defer below-the-fold assets or scripts that delay visual progress.
How to validate
Rerun the same lab audit and compare the filmstrip and Speed Index while checking that LCP and FCP also move in the right direction.
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
Run the free audit, then ask: “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.