In practice (I write Scala every day) compilation speed is a problem. I just recompiled a work projec, just under 28kloc. 124 seconds is a problem.
Admittedly, we do make heavy use of the type system.
Admittedly, we do make heavy use of the type system.
The project I'm working on right now is about 15kloc, but it's split in 3 sub-projects. Coupled with incremental (and continuous) compilation, this compilation speed issue hasn't been a problem for me.
Also, if you look around, many open-source projects built with Scala follow the same technique. It's quite sane too.
Incremental compilation suffers from the fragile base class problem. In a well structured app with proper encapsulation and rigid (and type annotated) interfaces, it helps a lot.
For example, if you delete AbstractFooBase.unusedMethod, sbt will recompile every file everywhere that references a concrete Foo instance.
(In principle it should only need to recompile files which reference unusedMethod, but it's not that smart yet.)