We have been holding back from moving to Scala 2.9 for a long time now. The reason is that Scala builds for Android need to be treated with Proguard to reduce their size. And on Scala 2.9, Proguard is incredibly slow. I think this has to do with the trait bloat that was creeping up on Scala library while it gained more and more features. Now that it's "fixed" by introducing more abstract classes to Scala 2.10 I was interested in seeing if my guess was correct and if the proguard performance would back to 2.8 level.
After fixing some issues in our build introduced by Scala 2.10, I run our build for Scala versions 2.8.2, 2.9.1 and 2.10.0-M2 (the currently latest milestone release). Timing information was gained from Lasse Koskela's maven-build-utils which I've done some work with lately.
Our build includes 7346 lines of Scala and some Java. Here's the performance information: