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During the past year, we’ve found and fixed a lot of perf issues in Expression and WPF.  I’d like to relate a few of them, not so much because you’ll have the exact same problems, but that the pattern of finding and resolving the bugs may be helpful experiences.  To start:

Expression uses a tree data structure to keep track of the XAML source code.  Each node in the tree has a property called Children that returns a List of all its child nodes. For leaf nodes this collection was always empty, but even so a new collection object was always allocated. In some scenarios, this resulted in megabytes of allocations.  To fix this, we implemented a singleton EmptyList class.  Leaf nodes always return the same instance of this collection, removing all the allocation costs.

• Browse the visual tree of running WPF applications.
• Inspect properties of elements at runtime.
  
• Edit properties of elements at runtime.
• Inspect RoutedEvents that are occurring, including the elements that handle them.
• Magnify sections of the UI.
• Find and debug binding errors.

My colleague Pete has it up on his site:  http://www.blois.us/Snoop/

This developed out of internal tools originally built into Expression that help us inspect our visual tree.  Originally we dumped the tree to a text file, then Pete put some UI on it, then Kenny incorporated the UI into Expression, and since then Pete has raised the stakes repeatedly making it standalone and adding features such as being able to inspect and even set values in the tree, and to be able to set breakpoints on data changes. 

Yes, you can debug databinding in WPF using Snoop.