I've tried all versions of Ubuntu, and Lubuntu has to be my favourite. My god is my netbook ever fast on Lubuntu (it uses LXDE gui).
I tried Linux Mint (Olivia, with Cinnamon GUI), on my desktop gaming PC, and it was just too damn slow. I have no idea why it was so slow, since my PC is more than capable enough for any operating system. It must have been running in 2D-mode, or didn't have HW acceleration turned on, or something. But the Linux Mint forums are completely dead, and there wasn't anyone there willing to reply to my posts about the issue.
Just a wild guess, but did you try asking on here? Being that Mint is based off of Ubuntu, I'm sure we could help you.