In my Ubuntu 12.04 sony vaio when trying to download packages from Ubuntu repositories wifi speed is very slow. Internet works fine for other downloads/browsing. I tried with changing the download server but no change. Getting a download speed <1kbps.
One of these solutions may work: http://linuxplained.com/how-to-fix-wireless-problems-in-ubuntu-1204-precise-pangolin/
It worked for me.