Just a quick note to myself (and others that may stumble across this blog entry via a web search): If a WPF application is running slow inside the debugger of Visual Studio 2010, but perfectly fine without a debugger (e.g. by hitting Ctrl-F5), then the reason may be Intellitrace.
In my case switching off Intellitrace (only available in the Ultimate Edition of Visual Studio 2010) helped gitting rid of the sluggish behavior of a DataGrid. In the “Tools” menu select “Options”, on the Options dialog click “Intellitrace” and then uncheck “Enable Intellitrace”.
Note that I do not have access to Visual Studio 2012 at the time of this writing, thus I cannot make a statement about its debugging behavior.