Performance and virtualized collections

Fabulous creates immutable widget descriptions, then compares the previous and next trees. Reusable native controls receive only changed scalar, child, collection, and environment attributes. Performance therefore depends more on stable structure and appropriate controls than on avoiding ordinary widget allocation.

Keep expensive computation and I/O out of view. Precompute data in update, preserve unchanged model values, and split large screens into components so local state does not redraw unrelated descriptions. Avoid dispatch loops caused by treating programmatic property updates as user input. Measure Release builds on representative hardware before adding caches.

Collections

Do not render a large or unbounded data set as hundreds of children in VStack. Use a backend virtualized control and a template:

• MAUI: CollectionView for most lists and grids. See the current builder in CollectionView.fs.
• Avalonia: ListBox/ItemsControl with a virtualizing panel, or the ItemsRepeater extension for custom layouts. See the compiled ItemsRepeater sample.

Keep item identity stable, make templates inexpensive, and page remote data instead of materializing it in view. Use observable collections only when the native collection is intentionally updated in place; otherwise update immutable model data and let reconciliation apply collection differences.

The reconciliation hot path is benchmarked in Fabulous.Benchmarks. Run it with dotnet run -c Release --project src/neutral/Fabulous.Benchmarks/Fabulous.Benchmarks.fsproj when changing framework internals, not as an application startup benchmark.