Optimizing the performance of XAML-based user interfaces involves identifying and resolving any performance bottlenecks in the UI. To do this, XAML developers should have experience with profiling and debugging tools such as Visual Studio, which can help identify performance issues and suggest solutions.
They should also have a strong understanding of performance optimization techniques, such as reducing the number of visuals on the screen and using virtualization techniques. Additionally, you may want to look for XAML developers with experience working with the latest hardware acceleration technologies, such as DirectX and GPU acceleration.