See also

    Improved incremental compilation of #scala in the
    making yields 10-40x compilation speedups:
    https://github.com/sbt/sbt/issues/1104#issuecomment-188529825

and

    After turning on the java8 backend in Scala, our 
    bytecode size decreased by 50% and compile times
    decreased by 75%. Amazeballs!

by some Scala devs.