Style recalculation on a page with 10,000 DOM nodes takes ~180ms on a budget Android — 10x the entire 16.6ms frame budget. The exact same scroll that is imperceptible on a developer MacBook drops 90% of frames on a Redmi Note 9 or Samsung A-series device. Why desktop profiling is misleading: The V8 engine on a budget Android runs at 5–10x lower throughput than on a developer laptop. Chrome's 6x CPU throttle preset is still optimistic for real low-end hardware. The only reliable signal is remote debugging on physical budget devices. What this covers: DOM size and style recalculation benchmarks on real hardware, content-visibility: auto , CSS contain , passive touch event listeners, IntersectionObserver vs scroll events, and the full mobile debugging workflow. The mobile hardware reality Before getting into fixes, it's worth internalizing exactly why budget phones are so much slower not as an abstraction, but as a design constraint.…