Improve Speed Index without touching LCP

Speed Index measures how fast the visible area fills, frame by frame — you can pass LCP yet still feel slow.

— Step 1: Lighthouse filmstrip + Performance screenshots show the fill curve.
— Step 2: Paint above-fold structure early: inline critical CSS, avoid client-rendered skeletons that pop late.
— Step 3: Stream HTML (don't buffer the whole page server-side) so paint starts at first chunk.
— Step 4: Defer below-fold images/sections so the viewport fills first.
— Step 5: Re-check Speed Index alongside FCP.

Example: SSR streaming + critical CSS: Speed Index 4.1s -> 2.3s with LCP unchanged.

Takeaway: aim Speed Index under 3.4s — it captures perceived speed LCP misses.