If you haven't yet, and if you care about the performance of your apps, take a look at the Optimizing WPF Application Performance document. It's got a ton of great perf tips and information about the inner-workings of the platform.
As a side note, we plan to add more links to the relevant performance tips from the docs so that they are easier to discover.
As always, do let us know if you have any questions or feedback on the docs or the perf tools,
We are the Windows Presentation Foundation SDK writers and editors.