When or if you run into trouble with apt-get and IPv6 connections timing out or not resolving properly at all, it may be a good idea to simply prevent apt-get from using IPv6.
Use
-o Acquire::ForceIPv4=true
when running apt-get, or create /etc/apt/apt.conf.d/99force-ipv4 and put
Acquire::ForceIPv4 "true"
in it.
If this does not work for you, you may want to have a look at /etc/gai.conf (this will, however, affect your system on a deeper level for IPv4 vs IPv6 connectivity). If you’re not interested in IPv6, it should cause no problems.
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