<p>Is Visual Studio written in .NET Windows Forms? Or is Windows Forms just too slow for a editor type application?</p>

Based on my experience with the VS SDK, it looks as if it was built with C++/COM but I think that's just because the Visual Studio team decided to go with that (AND Windows Forms didn't exist as a release version when they were first developing the VS.NET IDE).
You can use Windows Forms for an editor type application. I think SharpDevelop is a WinForms app.