Enlightenment is pretty fast. It's also extremely customizable, and potentially good looking. Might not be what you are looking for, but I think it deserves more attention. I am also trying out xfce at the moment. So far, I am very satisfied, but there are some bugs I have to sort out before it's usable.

Last time I tried lubuntu, it was not nearly good enough for everyday, or non-geek usage. I frequently had to retreat to command line to do stuff. I like their intentions, though, but I wouldn't recommend it for everyday use unless you've got a really slow computer or until they sort out some of the issues.