The Scala compiler is indeed slow, mostly because it has to do a lot more work than the Java or Go compiler. However, in my experience Sbt's incremental compilation works well for small to medium sized projects. Beyond that we need a bigger hammer, and we're working on a parallel (and later, distributed) Scala compiler [1].
Full disclosure: I'm one of the founders.
[1] https://triplequote.com/hydra.html