I was thinking that with little effort portage could use separate threads to download files and compile them, this could have some advantages, for example, when downloads are complete, you can go safely offline and continue compiling. I don't know if this exists, but if diferent instances of emerge are running at the same time, for instance, emerging two packages with some common dependencies, this feature will be useful.
*** This bug has been marked as a duplicate of 1661 ***