There are people with more technical knowledge than I who can answer this in more detail I'm sure. However, I *can* say that I've heard it's to do with the many background processes that are going on when Android is in use.
Because pure Mango mode avoids this, the signals are cleaner (less noise), so the sound is improved as a result. Maybe this results in a darker background.
I only use pure Mango mode, so I can't compare it with Mango in Android, but I can say that I don't find it to be excessively laggy in pure Mango mode; but everyone will have their own standards and expectations.
Hope this helps a bit