Mint might also be a little bit lighter/faster than Ubuntu. Mint's other variations(Mint LXDE, Mint XFCE, Mint Fluxbox) are all definitely lighter/faster than Ubuntu.
Personally, I recently switched to Lubuntu(use the LXDE desktop instead of Gnome, and comes with minimal apps installed, and all of these apps are incredibly light). And the distro is blazing fast. I have a seven year old computer with only 512MB of RAM(which I'll probably upgrade to a GB soon), and because of how bloated Ubuntu is, my computer would sometimes slow down to almost a crawl. I used Ubuntu for over three years. And everytime I installed it the distro would be really quick. But within a couple months time, everything slowed down. The distro just comes with too many apps installed, and most of those apps are heavy in the memory footprint department.
So depending on your computer specs(which to me, I wouldn't care if I had 8GB of RAM and the fastest processor, I would still stick with Lubuntu) you might want to look at lighter/faster distros. Besides Lubuntu or Mint XFCE/Fluxbox(I found both to be faster than Mints LXDE, which I'm an LXDE fan, I just don't know why it's slow on Mint), you might want to try out Crunchbang or Peppermint. All of these you can throw on a LiveCD and check them out before installing them. Arch Linux is supposed to be really light and fast too. But I haven't tried it out yet. Also, Arch isn't an Ubuntu derivative.
But as far as Ubuntu derivatives go, Lubuntu is definitely the lightest. And with release 10.10, there's going to be a minimal install, so it's going to be even lighter!