If the first mirror of GENTOO_MIRRORS is down, Portage will try to fetch from it 5 times (by default), then it will be skiped, and it will try to fetch from the next mirror. That's OK, but when portage wants to fetch another file it will try with the first mirror AGAIN. So, if portage wants to fetch 10 files, it will try to fetch from a down server 50 times. It's a bit annoying. I know it's not a major enhacement but it's a detail that could be taken into consideration.
Maybe if someone provides a patch.
*** Bug 141760 has been marked as a duplicate of this bug. ***