I got an idea after reading bug 18111 to decrease the install time when doing 'emerge system' and also other emerges with have multiple files. When you start the 'emerge system' process you should have an option to run an 'emerge -f system' in the background. So when the first downloaded file is being compiled the rest of the packages are downloaded in the background. This could be something like 'emerge -F system' which stands for 'Fetch while compiling'
sounds more like a portage request
Looks like a duplicate of bug 1661 to me.
*** This bug has been marked as a duplicate of 1661 ***