Slow download speed (ONLY downloads from ubuntu main server) (from Japan)

Recently my download speed from Ubuntu servers (ip starting with 91.189…) is very slow.

Previously (July 2016) my normal speed from the main server was 5-12MiB/s (from updates and from cdimage) but recently (from September 2016) I often only get 20KiB/s-200KiB/s. Sometimes it goes back up to my previous normal speed (5-12MiB/s) but this usually does not last long (1-10 minutes at a time). Whether my download is fast or slow seems to be completely random. This slow speed applies to any download from ubuntu including cdimage, synaptic, and update manager (they all connect to ip 91.189…).

I consistently get 9-15MiB/s from cdimage.debian.org and 50MiB/s from iso and update mirrors in Japan, so I think it is probably not a problem with my internet. I live in Tokyo Japan and have a gigabit connection.

-disabling ipv6 does not help
-I have a skylake i5 6600 cpu. Switching motherboard (msi to gigabyte h170) did not help. Both motherboards use Intel i219v LAN adapter.
-the computer I used in July was a haswell desktop, it is broken so I cannot test it. I bought the skylake desktop in September.
-Buying a new router did not help
-upgrading from 16.04 to 16.10 did not help
-I do not know exactly when this started because I did not use the internet between July and September 2016

This seems like something you might mention on the main Ubuntu forums?
I doubt anyone here would be in a position to help if it’s a server-side issue, that’s canonical’s turf.

Look at your router’s info about attenuation rate and other and log them at different times thru the whole week and post here the info.