INP 480ms -> 150ms on an e-commerce PLP

INP 480ms -> 150ms on an e-commerce PLP
INP = Interaction to Next Paint, the p75 latency of clicks/taps. The 'add to cart' click ran a synchronous 220ms recommendation recompute on the main thread.
— Profiled the long task: 180ms in a reduce over 3,000 products.
— Moved the recompute into a debounced, post-paint requestIdleCallback.
— Long tasks during interaction: 2 -> 0.
p75 INP: 480 | 150
p95 INP: 910 | 260
Takeaway: the click stayed; the 220ms blocking task moved off the critical path.