The reason for why it hasn't been a problem for us is because we've been making heavy usage of SBT's support for sub-projects. Scala makes it easier than other languages to decouple components, so after a while you begin to spot possible sub-projects all over the place.

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.