Kotlin is not actually slow to compile. The problem is that incremental compilation and gradle/android don't mix. So it ends up recompiling the whole thing each time.

I use Kotlin for non-Android projects with Maven, so incremental compilation works. Build times are not a significant issue, though the Kotlin compiler is still not as fast as the Java compiler is. I noticed from doing hotswaps when debugging processes that Kotlin seems to still compile too much, even when in incremental model. So I hope once JetBrains get stability under control they can spend some time optimising.