With Debian you can easily auto-detect the fastest mirror for apt-get!
First install this package:
$ sudo apt-get install netselect-apt
Then run the following, adjusting for your desired version. I’m using LMDE, so I’m going to run:
$ sudo netselect-apt testing
sudo netselect-apt [stable|testing|unstable|experimental|release_codename|sid]
This will query various debian mirrors then leave a file sources.list in your current working directory.
You can now manually update your /etc/apt/sources.list with the fastest mirror found for your system.
$ sudo nano /etc/apt/sources.list
Only replace the existing line
deb http://[___]debian.org/debian stable main contrib
with your new mirror. Leave others such as security.debian.org and third party repos as they are.