Big programming languages are built from big JARs (compiler, runtime, stdlib). Loading those each time in memory is a long process. Using nailgun saves ~10 seconds on my laptop.

Another problem is compiler performance (both Kotlin and Scala compilers compile less statements per second than Javac), but it's not that dramatic.

Finally, dexing languages with runtime (kotlin - ~200KB, scala - more than 1MB) takes time.

Luckily, Buck optimizes most of it.