I have an app that is slow to start. I assumed it was because of all the various controls on the first page, but even after I removed all the XAML (as an experiment), my start time wasn't reduced (or at least not perceptibly). I then noticed all the various assemblies getting loaded one by one in the output window.
Obviously, adding anything to a phone application (with its limited resources) is going to affect performance. But at what point will adding dlls start affecting start up performance? If you create a new app in Visual Studio, you'll have ten references automatically. How many more until you have a performance issue?