Edge is a great browser engine and is really high performance now. It's comparable to Chrome in the latest releases.
If XB1 is slow because of deliberate limitations by Microsoft - e.g. capping CPU usage - then there's nothing much we can do about that. It's not Construct's fault, and no other framework can get around that either, it's the XB1 OS that is capping apps that run on it. In that sense UWP apps will run exactly as well as Microsoft let them. This isn't anything to do with the performance of C2 or HTML5. It's the same thing if an app can only use 128mb RAM (which IIRC I've not seen any official reference for, would appreciate a link on that) - no other framework or technology is going to let you get around that, it's an XB1 issue, not a Construct issue. So the best people to go to about this would be Microsoft. FWIW you should already be able to detect backgrounded games (Browser plugin 'On suspended', 'On resumed', same as all other platforms), and you can try using the savegame system to do full state saves and restores to work around that too.