The server selection is only for the Mint repos which just contain a small part of the packages. The majority comes from Ubuntu. Those servers can be changed manually: on the Others tab, select Ubuntu and press Edit, then change the URI with one from a mirror from this list: https://launchpad.net/ubuntu/+archivemirrors
Then please do the same with "Recommended Updates" which is also from Ubuntu.
Registered Linux User #528502
Feel free to correct me if I'm trying to write in Spanish, French or German.