Rebuild a Lighthouse perf score from 40 to 90

Rebuild a Lighthouse perf score from 40 to 90
The Performance score is a weighted blend, not one metric. Spend effort where the weights are.

— Weights (Lighthouse 10/11): TBT 30%, LCP 25%, CLS 25%, FCP 10%, Speed Index 10%.
— Step 1: Fix TBT + LCP first — they own 55% of the score.
— Step 2: TBT: break long tasks > 50ms, defer JS. LCP: preload hero, cut TTFB.
— Step 3: CLS is cheap points — size every image. Often +10 alone.
— Step 4: Run 5 times, take the median; one run swings 8+ points.

Example path: 42 -> 71 (TBT) -> 84 (LCP) -> 91 (CLS).

Takeaway: 55% of your score lives in TBT + LCP — start there.