Hi, I've been having the same issue
with SuSE 9.3. The problem was caused by the 'fam' (File Alteration Monitor) daemon.
I had a quick look on Mandrake/Mandriva's package list
and it looks like Mandrake also uses fam.
Try opening 'ksysguard' or hit [Ctrl]+[Escape] when you next login, and when things start going slow have a look at fam's system resources. Mine was climbing all the time to 75-90% and was causing apps to not load or lock up.
Killing fam fixed the problem and I haven't noticed any loss of functionality yet when fam isn't running.
I was able to disable fam on bootup by opening up a root console and doing a 'chkconfig -s fam off'.