Enter netselect-apt, a package which helps one to find and use the fastest Debian mirror for his region.
Read about it here
What this script does in conjunction with 'netselect' command is, it first downloads a list of all the Debian apt mirrors from the official Debian website using wget. The list is saved in a file named 'mirrors_full' in your current directory. It then checks the responsiveness of each of the servers in the list and then selects the fastest among them. Finally it writes the necessary code in the sources.list file and saves it in your current directory. Considering that on last count there are a total of 248 Debian mirrors to choose from, that is a pretty tedious job.