By default Mac OSX App Store Automatically checks for newly available Updates and downloads it in the Background and you are notified once the updates are downloaded and ready to be installed. So it may slow down your download/uploads if you have a slow broadband connection. If you do not want auto updates to happen,
1. Goto Spotlight Search and Type : System Preferences and Open it
2. Now Click on App Store Icon,
3. Deselect "Automatically Check for Updates"
Thats it!! Now updates would not be downloaded Automatically.