Speed Index Signals Test
The Speed Index Test measures how quickly the visible content of your page fills in during load — not just when the first or last pixel appears, but the average rate of visual progress across the entire above-the-fold a…
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- Pass / fail + fix guidance
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The Speed Index Test measures how quickly the visible content of your page fills in during load — not just when the first or last pixel appears, but the average rate of visual progress across the entire above-the-fold area. Google's "Good" threshold is under 3.4 seconds; over 5.8 seconds is "Poor". A high Speed Index means users see a partial page for too long, which feels broken even when everything eventually loads.
What This Tool Checks
- Speed Index timing in headless Chrome
- Filmstrip frames showing visual progress
- Above-the-fold content render order
- Image / font loading sequence
- Render-blocking resources delaying above-the-fold paint
- Server response time (TTFB) impact
Why It Matters for SEO
Speed Index captures perceived speed in a way single-point metrics like LCP and FCP cannot. A page can have an OK LCP but still feel slow because content fills in jerkily. Optimising Speed Index forces you to think about the entire load progression — what renders at 1 second, what renders at 2 seconds, what is still missing. Improvements here usually also improve LCP and FCP because the same render-blocking resources affect all three.
How to Fix It
Inline critical CSS so above-the-fold content can render immediately. Use font-display: swap on web fonts. Preload hero images. Defer all non-critical JavaScript. Improve TTFB at the origin if it exceeds 600 ms. Re-run until Speed Index drops below 3.4 s.
How It Works
Headless Chrome captures filmstrip frames during page load, analyses the visual progress of the above-the-fold area frame by frame, and computes the Speed Index as the average time at which visible content was painted. Slow-painting elements are flagged with the resource that delayed them.
Common Mistakes to Avoid
- Above-the-fold content waiting on web fonts (FOIT)
- Hero images loading in a single late request
- Render-blocking CSS in the <head>
- JavaScript executing before above-the-fold paint
- Slow TTFB pushing every metric back simultaneously
Quick Checklist
- Speed Index under 3.4 seconds
- Critical above-the-fold CSS inlined
- Web fonts use font-display: swap
- Hero image preloaded
- No JavaScript blocking above-the-fold paint
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A composite metric that captures how quickly above-the-fold content visually fills in during page load. Lower is better; under 3.4 s is "Good".
LCP is a Core Web Vital and a direct ranking input. Speed Index is a Lighthouse-only metric but better captures perceived load smoothness. Optimise LCP first, then use Speed Index to fine-tune.
Lighthouse captures filmstrip frames during load, computes the visible completeness of each frame, then integrates over time. Faster visual progress yields a lower (better) score.
Render-blocking resources in the <head> are the biggest cause of poor Speed Index. Inlining critical CSS and deferring JavaScript almost always improves the score significantly.
No. It is reported in Lighthouse but is not part of Google's ranking signal. LCP, INP and CLS are the three official Core Web Vitals.