Sometimes even after the preview window has finished updating, tg2 keeps using the cpu at full tilt until you shut it down.
BTW, although I'm sure it's in the works, multithreading is probably the most sought after new feature that could be implemented ATM.
At random it also appears to carry on running at 100% even after the program is shut down.
I haven't seen it continue even after the app is gone. That seems very odd. I suppose you mean it's not shutting down properly at all?
The issue with the preview/app sometimes continuing to use CPU even after it's done updating is a known bug. I'm not sure where it is in the fix priority queue, but it'll definitely be addressed.