It might not be our code that is causing slowdowns. Our SyntaxEditor control (the code editor) uses the same file loading code as our WPF/Silverlight versions and there we can load and display very large files almost instantly there. I just did a test of Code Writer on a 10MB C# file and it opened it right away too.
I suspect if you see any slowdowns in loading, its due to the Microsoft WinRT API communicating with your storage device. Such as a file on SkyDrive might not yet be cached locally and would take time to download. But on a local hard drive, it should load everything pretty zippy.
For scrolling performance, we're stuck a bit there. The problem is that Microsoft doesn't give us any way to draw text in a custom control other than to add a ton of TextBlock controls, one for each segment run. The process of using those to measure and render the text is what causes the slowdown. In the WPF programming platform, we have access to a rich text drawing API and everything is super fast there with our control. We have requested to Microsoft that a similar API be added in the future to WinRT (Windows 8). We'll have to wait and see if they do since that would fix this.