Scaladoc issue documented:

https://github.com/scala/scala/issues/2467

Also I think it'd be best to include a FIXME relating to a commented out portion of code so that we know to come back to this.

Interestingly I see that we were using a ThreadPoolExecutor for internal execution context. FJP has been cited to have performance benefits over TPE so perhaps this is why we're seeing some dramatic improvements. I shall investigate further using another PR.