Mac OS X Lion too slow

More Info Please.

Year, screen size, CPU speed, and amount of RAM installed in your iMac?

How full is your Hard drive?

Do you have any antivirus software installed?

Do you have any "crapware" like MacKeeper installed?

Explain what is "slow"?

You may find some useful information here:

https://discussions.apple.com/docs/DOC-3521

Ciao.