Disable the WPF designer in VS 2010?
I don't know about 2010, but in 2008 you can Tools->Options->Text Editor->XAML->MIscellaneous->"Always open documents in full XAML view"
Maybe this helps.
I haven't tried this in Visual Studio 2010, but in 2008 you can have it treat XAML files as XML files (and thus use the standard XML text editor, rather than the designer's XML editor). You can do this by right clicking an XAML file, select "Open With", pick XML file from the list, and then set that as the default.
At least in 2008, you still got pretty good intellisense, but none of the overhead of the XAML designer.
Try turning off "Automatically Populate Toolbox" in addition to Open in XAML View (it's on the same dialog). This seems to really improve the performance as well.