I thought that maybe it will be a good idea to start parallel download of packages during compilation of previous ones. E.g. now I download mozilla-firefox (it takes 30min) - why my portage can't compile previous package at this moment? Then, when it will start to compile mozilla-firefox, why it can't download something in background? It would save the time.
Sorry, I cut sentence by mistake: Of course it would be OK for doing emerge for more than 1 package.
man make.conf: FEATURES="parallel-fetch"