Seems fine on mine. I'd recommend a clean install if you're having issues. Im sure that's come across your mind already though. I only trust clean installs myself.
Normally I'd think of that, but AFAIK I would have to install Leopard, then the Lion upgrade, then the Snow Leopard upgrade. Is there a way I can just wiped the drive and install Snow Leopard?
Just copy your DMG for Mountain Lion to a USB drive or DVD and boot from it. Google if you need help
I think I might try that. Since I installed it, I guess I have to re-download it in order to copy it :-(
EDIT: Do you boot from a USB stick on the Mac by holding down a certain button like you do for booting off DVD?
Before you do what could very well be a useless act (reinstalling), could you tell us WHAT is slowing your Mac? What processes?