@Ichoran 

The other pull requests will be trickling in. I've been throwing a lot of ideas around in https://github.com/retronym/scala/compare/topic/opt2 and will be working to find what's worthwhile in there with @gkossakowski next week.

On that branch, I saw improvements in the order of 10-15% of a warmed up resident compiler FSC repeatedly processing `src/library/util/**.scala`. But the usual caveats apply; that result might not generalize, and I might have made things incorrect along the way.

Some (many) of the commits there seemed to deliver cleaner YourKit profiles without real improvements in elapsed time.

Some profile results suprised me (as usual).  A lot of time was reported in the constructors of `AbstractSeq` and friends. I ended up writing a custom `ListBuffer` that didn't participate in the collections hierarchy at all to, and use that in a custom version of `map` that was specialized for `List` (no allocations on Nil.map, no indirection through the CanBuildFrom).

As soon as we figure out which changes seem to be the most beneficial, I'll let you know.