I also highly recommend Anvil. Anvil + Kotlin makes Android development feel sane and at times even joyous! I should write a blog about my experiences.

It depends how you use it.

At the moment the gradle plugin doesn't do incremental compilation. If you use it for Android, that's an issue because of the Android build system.

I use Kotlin for free standing programs that run on my laptop/desktop. In these cases it's about as fast as Java, but that's because IntelliJ controls the compilation itself and gradle/maven is used only rarely.