Chuckj on XAML conditional compilation ... a must read ...
Rob Relyea points to Chuck Jazdzewski' s post on conditional compilation ..
IMO, this is one of the top features you need for troubleshooting perf in WPF . At some point you will be thinking "what would happen if I used ImageBrushes instead of cached VisualBrushes?" or what happens if this template is lighter, etc.. the XAML gets messy very quick with out this awesome feature ... [trust me I was there] ...
Apologies for surfacing an 'old' post that I missed before [ I do subscribe to Chuck's RSS] ... but this is so useful that I could not help pointing to.. [while at Chuck's check the rest of his XAML series]