Some mirror tiers are huge. For example, mirror://apache lists 114 mirrors. Now, if a distfile listed as mirror://apache did not get uploaded to the mirrors yet, it's quite likely Portage will try the 114 mirrors in order. Some of them dead, some of them terribly slow.
Long story short, 20 minutes wasted trying mirrors that don't have the file. I think it would be better if Portage tried N randomly chosen mirrors from a tier and then tried other possible URIs.