As long as the users want more and more apps very cheaply they might have to accept the performance they get. It has also been a problem that most OS's have different GUI frameworks which means you have to re-devlop every app/application multiple times. This is an enormous time sink. When all operating system uses the same GUI framework (and possibly the same languages) it might be very effective to use that instead of html+css. Until that it is still expensive and as I started with, the users want more for less.