A modern programming language that makes developers happier.

kotlinlang

Well, here's information from my current project.  According to github the codebase is 80% Java and 15% Kotlin.  This is with building from clean `./gradlew assembleFlavorDebug -profile` and not using gradle daemon.  The JSON holders are still in java.  This is only one sample, I don't know much it generalizes but perhaps it will help you make a decision.

```:app 	1m26.51s 	(total)
  :app:preDexFlavorDebug 	49.211s 	
  :app:compileFlavorDebugKotlin 	13.081s 	
  :app:mergeFlavorDebugResources 	11.812s 	
  :app:dexFlavorDebug 	6.762s 	
  :app:packageFlavorDebug 	2.293s 	
  :app:compileFlavorDebugJava 	1.747s```