OS X isn't like a PC that requires OS X to be re-installed every few months. Your feeling is probably psychological, however you could benefit from upgrading the RAM too. If you tend to spend a lot of time on RAM intensive apps that would cause a slowdown if you haven't upgraded the RAM.
However to answer your question, restart in Recovery Mode, start Disk Utility and do and Erase the HD, the exit DU and click install OS X.
Backup you stuff first! Then proceed as kaufmann recommended.
An easy check before doing any reinstalls is to create a new user account and operate from it for a bit to see if you still perceive the same system issues. You may need to simply refresh your user account.
I think it's most likely your perception. What are the specs of your iMac and what are you running on it that is slow? I would just wait and install the new OS when Mavericks is released in the next few weeks.