Scala incremental compiler improvements by Grzegorz Kossakowski. Actual runtimes are not so important here, since the two benchmarks were run on different machines, and also they make no attempt to ensure that the scala compiler is fully JITed, which during normal Play development offers significant improvements over these times as the compiler …

With Grzegorz's improvements

Initial clean compile of entire project:
[info] [info] Compiling 29 Scala sources and 1 Java source to /tmp/inc-compile/target/scala-2.10/classes...
[info] [success] Total time: 15 s, completed Aug 28, 2013 9:43:38 PM

Now simulate adding a method to a controller:
[info] [info] Compiling 1 Scala source to /tmp/inc-compile/target/scala-2.10/classes...
[info] [success] Total time: 2 s, completed Aug 28, 2013 9:43:40 PM

Now simulate adding a new route to the routes file:
[info] [info] Compiling 2 Scala sources and 1 Java source to /tmp/inc-compile/target/scala-2.10/classes...
[info] [success] Total time: 5 s, completed Aug 28, 2013 9:43:45 PM

With SBT 0.12

Initial clean compile of entire project:
[info] [info] Compiling 29 Scala sources and 1 Java source to /tmp/inc-compile/target/scala-2.10/classes...
[info] [success] Total time: 25 s, completed 29/08/2013 2:49:17 PM

Now simulate adding a method to a controller:
[info] [info] Compiling 1 Scala source to /tmp/inc-compile/target/scala-2.10/classes...
[info] [info] Compiling 29 Scala sources and 1 Java source to /tmp/inc-compile/target/scala-2.10/classes...
[info] [success] Total time: 16 s, completed 29/08/2013 2:49:32 PM

Now simulate adding a new route to the routes file:
[info] [info] Compiling 2 Scala sources and 1 Java source to /tmp/inc-compile/target/scala-2.10/classes...
[info] [info] Compiling 29 Scala sources and 1 Java source to /tmp/inc-compile/target/scala-2.10/classes...
[info] [success] Total time: 10 s, completed 29/08/2013 2:49:43 PM

I decided to make it easier to follow the status of the merge by creating a meta issue. Check it out here: sbt/sbt#1010

Awesome!