A single shared observable can kill scroll performance: one mutation, many ancestors re-render, stuttery scrolling and layout shifts.

• Read \(.sizeCategory) as close to the view that renders text and prefer semantic Font for textual content. Use @ScaledMetric for non-font geometry (icons, gaps) so those values scale with Dynamic Type.
• Replace global mutable scale objects with immutable props or localized observables to reduce invalidation fan‑out. Precompute heavy layout work outside View.body when possible.
• On mixed UIKit/SwiftUI surfaces, consider wrapping UIFontMetrics in narrow adapters and snapshotting views at representative DynamicTypeSize values to check visual parity during migration.
• Profile on device with Instruments and correlate traces using OSSignposter to find expensive layout work and verify fixes during incremental rollouts.

Choose adapters for parity during migration; move to Font + @ScaledMetric once the surface area is small and verified.

How have you instrumented view invalidation to find fan‑out in large SwiftUI lists?