Diagnosing scrolling jank

Reproduce the problem first on the same device class, using a Release build and a realistic amount of data. Note whether the jank is constant on every frame or spikes when new rows appear or images finish loading.

Profile before you guess. Attach a real device, open Instruments, and start with Time Profiler filtered to the main thread and sorted by weight. Look for main-thread stacks doing image decode, JSON parsing, regex, or repeated layout passes.

Most jank comes from one of four causes, and the trace tells you which:

1. 1.Layout (constraint thrash, long NSISEngine stacks): simplify constraints, split heavy cells, cache row heights, and remove layoutIfNeeded() storms.
2. 2.Rendering (offscreen layers, blur, shadows): reduce layer effects, or rasterize deliberately where the content is static.
3. 3.Images (large assets hitting the main thread): downsample to display size and decode off the main thread.
4. 4.Main-thread work (JSON parsing, disk I/O): move it into async/await pipelines and touch UI only on the main actor.

SwiftUI has its own version of this problem. Unstable identity in ForEach, or a row body that reads more state than it needs, rebuilds rows on every change and looks like layout jank. In UIKit the equivalent is cell reuse that rebuilds subviews every time the cell is configured instead of resetting them.

Task {
    let data = try await loader.data(for: url)
    let image = await decodeOffMain(data)  // CPU work off the main thread
    await MainActor.run { imageView.image = image }
}