I love programming with and for Windows Presentation Framework. Mostly I write browser-like apps using WPF and XAML.
But what really annoys me is the slowness of WPF
WPF is meant for computers with modern graphics cards. Do your clients have modern graphics cards capable of running Aero? If your clients have older graphics cards, WPF will fall back to software rendering which runs extremely slow in comparison to hardware accelerated graphics.
You also might want to profile your application to make sure that it is actually WPF that is the slow part. It's possible that there is something else that is actually the bottleneck.