From a MacMini G4 user, I have done the same thing, actually switched from Panther to Leopard. I did this because I felt that the Mini had slowed down considerably, too many Spin Balls of Death. At first, I did not a clean install but a partial install and I had the same slow down you note now. After reading some forums, most recommend a clean install. So, I did that. Even with a clean install, it still had problems with internet connectivity etc... noticably slower with operations. It took much patching but the Leopard install got much better after it "settled down." It took a couple of days for this to occur.
I really do not know what sort of install you did, but I recommend a total clean one. Leopard might be good but it is more resource hugging then both Panther and Tiger.
I do notice that my laptop, with the same sort of chip as my MacMini G4 seem to be a better machine, without a slow down but I still have Tiger installed in it. Never had to reinstall anything, it still has the original install that is almost 4 years old.