As Jetbrains team improving kotlin compiler continuously over the last 4 years, it became much way faster than it was before.

this is true if we talk particularly about incremental builds by introducing much more optimizations like KAPT incremental...etc, this helped incremental builds to be much more significantly faster and for us developers as we run incremental builds very often.

though in clean builds maybe java is still the dominant one(I'm not sure) but:

1- You run clean builds rarely

2- even when java is outstanding than kotlin, the difference isn't that great.

With those in mind, I think we don't have to worry about large kotlin build times anymore.