Maybe they are slower to some extend, but for most cases it won't matter too much. I wrote simple game in scala and it was easy to constantly get 55-60 fps on newer devices: https://github.com/pkukielka/stronghold-defense
That being said I agree that jars are bigger and scala generates much more anonymous classes. Even with ProGuard that could be a problem on Android 2.3 or older. And ProGuard is a bit slow (for my game it was taking 30+ seconds for every build). Also Typesafe doesn't have plans for making scala first class citizen of Android.
So while Android development with scala is certainly possible there are few inconveniences which one should be aware of.
Maybe this will improve with ART.