One thing I've found helps a LOT is encapsulation and return type annotation.

If you change a public method, SBT will recompile all users of that class even if they don't use that method. If you change a private method, it's smart enough to recompile only the one file.

Annotating your return types both saves the compiler the effort of figuring out what you meant, and also prevents you from accidentally changing the return type and forcing recompilation of a bunch of client code.