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.

Unfortunately, not. We had a bad gradle config for a while so when we fixed that our build time jumped down from 6min incremental to 2min. However, I can tell you that I haven't noticed an increase in build time since we fixed our gradle settings which happened a few months ago and we have added code. Maybe it's a function of gradle modules (we have around 25)? If we ever spend a bunch more time trying to optimize our build time we'll write about it somewhere.