Haha I would hardly consider Ubuntu too be "lightweight". As one poster said about trimming down a Mandrake install, I would tend to disagree. A lot of times the core libraries aren't compiled to be stable / fast, perhaps they are a fairly generic (bloated) compile job. Just cutting out processes and excess software should help a bit, but you won't be touching a true lightweight distro even with a trimmed Mandrake (or SuSE, or whatever). There are core choices made that don't focus on speed, and so heavyweight distros could probably only get trimmed down to "medium weight"
PS: I've found RPM based distros slow too, but then again I'm biased and dislike RPM