If you have ever used linux mint, you will know its almost identical to ubuntu in every aspect, instead of a few asthetic changes. Now its a bit more than that, but a bulk of its software, the way it is configured, and a majority of the repo's for Mint come from ubuntu, and they rely on Ubuntu's main repository for you to get more software choices and update.
Now ubuntu's main server can be quite fast at times, but Ubuntu has tons of mirrors, and apt and synaptic are capable of searching for the fastest one for you to use, with Mint this is not possible, cause they give you only access to the list of Mint's repositories that doesn't actually contain much of the software you are gonna want or will update from.
The other problem with the way Mint does this, since its basically Ubuntu, you can use Ubuntu PPA's, without much concern for compatibility issues, Mint is about 99% compatible with Ubuntu atm. The problem happens when you got to add a PPA, it will put Mint's release codename, and PPA's are setup for Ubuntu's codename's. Yes you can go in and manually change this around, but this grows tedious and annoying, Since having Mint's codename serves no useful purpose.
Here's how to change things, so you can have access to ubuntu's mirror network to get the fastest possible downloads from their repositories and PPA's set themsevles automatically correct. Open up a terminal:
sudo gedit /etc/lsb-release
This will open up a file containing this information:
DISTRIB_DESCRIPTION="Linux Mint 12 Lisa"
you will need to alter it the information to the corresponding Ubuntu information that this release is modeled after:
save, reload synapitc, and go to software sources, you should now be able to access Ubuntu's mirror list and have it find the fastest server for you, and now when you add a PPA, it should add it correctly. Hope this helps folks.