I've been seeing this all day today and finally tracked it down. The gentoo.ussg.indiana.edu mirror has some sort of issue/limitation at the moment and should be taken out of the distfiles.g.o rotation until they fix it. I am consistently getting 1MB/s speeds from osuosl but only 80-100K/s speeds from gentoo.ussg.indiana.edu. This is sustained speeds and easily reproducible by trying to grab a large file directly from that mirror. thx.
It's reported to be much better today. Closing for now, reopen if it becomes a problem again.