Key word "supposed".
Try installing apt-spy (3.1-13)
writes a sources.list file based on bandwidth tests
Parses a list of mirrors and tests each of the mirrors for bandwidth. Writes a /etc/apt/sources.list file based on the responses it gets.
If that doesn't help look at
Debian mirrors and find a mirror closer to where you live. Although closer is not always better. Try couple different mirrors.