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 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