Speed Index

What it is

Speed Index — originally from WebPageTest, now also computed by Lighthouse. The tool records a video of the page load, computes per-frame visual completeness, and integrates over time. Lower is better; thresholds are mobile ≤3.4s green, desktop ≤1.3s green.

Why it matters

LCP captures one element; Speed Index captures the entire above-fold paint experience. Two pages with the same LCP can have very different Speed Index — the one that renders progressively (text first, image after) feels faster than the one that holds back. Useful as a tiebreaker when LCP is acceptable but `feel` isn't.

Who it applies to

Sites optimizing perceived performance, not just numerical milestones.

How WQI scores it

Web Quality Index considers this standard satisfied when the supporting factor passes.

Related standards

See also

CWV , LCP

Standards that share factors with this one

Auto-computed from overlapping factor tickets in satisfiedBy, excluding standards already listed under "See also" above. Strong overlap suggests these standards rise and fall together when sites are scored.

• Mobile viewport meta tag 1 shared factor SEO `<meta name="viewport" content="width=device-width, initial-scale=1">` — one line that decides whether your site is usable on a phone. Mobile-first indexing means without it you don't rank.
• Resource hints (preconnect / preload / prefetch / dns-prefetch) 1 shared factor Performance Tell the browser which connections, fonts, scripts, and next-pages it should warm up before they're strictly needed. Single-digit-percent LCP improvements per hint, multiplied across a page.

Other references

• tooling WebPageTest — Speed Index docs