Why the New Android Runtime (ART) does not compile all of the methods in app at default?

[王帅]

I am interested in the ART of Android 6.0. But it seems that it has several compilation options to choose for compiling apps such as everything, interpret-only, speed and so on. 
The reason why google replace DVM with ART is due to its Ahead of time compilation. It is reasonable that an app will have the best performance if all of its methods are compiled. 
But the default mode of ART is "speed" not "everything". Why ?  Thanks a lot.


I have tested on some apps. It shows that if ART compiles all of methods of an app, it will perform better.