Scala's compile speed (with incremental compiles) is generally acceptable if you have reasonable dependency patterns. If you have cyclic dependencies, it's a problem. However, cyclic dependencies are signs of things that are wrong in development practices that can't be blamed on the language.

You mentioned flight control systems: those are unlikely to be written in Scala, being real-time systems, but if one were, it would not need to be compiled and recompiled on the fly.

Scala's a great language but it's not the right language for every problem. No language is.