Surely it is not the distribution?
Though Slackware or the Slackware derived Vector, might be a bit faster.
Surely its the Window Manager? On slow machines, fvwm or Windowmaker should run fine regardless of what distro, and kde or gnome should be painful similarly regardless of distro. Fluxbox or blackbox should be very fast. Or maybe even enlightenment?
I found that WindowMaker and the others mentioned would run fine on Debian on a 200mhz mmx with 64Mb memory. Nothing with KDE or Gnome would even boot. KOffice would run fine, but OO was painful. Konqueror was fine too, though xfe was snappier as a file manager.
If you want to test before you buy, boot with DSL. If DSL works well, then Ubuntu with a light WM will most probably work too. There are isos of e-live available also, if you want to test enlightenment.
Peter