For a fast upgrade, I've downloaded the alternate CD using torrents. After the upgrade in this way, I still ended up with a system which has still not all updates (probably after the ISO was packed).
After a quick search, I ended up on the list of mirrors on Launchpad. Search for your country and test some. I live in the Netherlands and got only 75kB/s speed, much lower than my usual speed. The ubuntu.mirror.cambrium.nl mirror did not work for me, but ubuntu.mirror.astratoip.net did. Replace the mirror accordingly and run the below commands:
sudo sed 'firstname.lastname@example.org@ubuntu.mirror.atratoip.net@' -i /etc/apt/sources.list sudo apt-get update
archive.ubuntu.com by your current mirror.