We have a WinJS app that is not performing as well as it would if it were written in XAML/C#. Is it possible to move the app into XAML, piece by piece, by either hosting XAML inside the WinJS or hosting WinJS in XAML? Or is a complete re-write necessary?
Rewrite. Hosting XAML inside a WinJS app is impossible, and hosting WinJS inside a XAML web view would be extremely difficult - the communcation across the two technology stacks would be difficult enough that you wouldn't save anything time or effort wise.