I've only looked at Kubuntu in the sense that I installed Ubuntu and then got KDE with apt (something like apt get kubuntu desktop??). And I thought it was okay, but then it is quite hard to find a Linux distro which has nothing to recommend it nowadays.
I can't say have noticed MCC being especially slow (but then it is an old computer anyway, so I expect it to be slow), nor have I tried kooka on Mandriva, as my scanner is attached to a SuSE box and I prefer Xsane (don't know if it works with OCR as I mostly use it for scanning photos).
Where Mandriva 2006 has worked best for me is in updating, the GUI version of update actually works! With 2005LE I frequently found that it would report that package signatures had failed, but this was because only the file headers, not the complete packages had been downloaded. Also KDE start-up, while not quick anyway, would often hang for unreasonable amounts of time, this is not a problem I have had with 2006.