Scenes and memory
“How big is, in a terms of application performance, the benefit of making different scenes (for levels etc) – compared with having for example, all levels in the same scene?”
A scene and its dependent resources are only loaded when using the function “application.setCurrentUserScene”. Dividing a game into scenes will help to minimize the memory footprint of your game, and will also make some differences performance wise. Even if the engine will only do heavy calculations for visible/active objects, there will be some little overhead identifying them each frame.