What model iMac do you have (e.g. early 2008 model)? How much RAM memory do you have installed on it? Although Mountain Lion will run on a 2GB iMac, it's really a dog until you have at least 4GB of RAM.
Did you JUST install Mountain Lion? If so, it's possible Spotlight is still indexing your hard drive, which can take quite a long time to complete (and will slow things down until it's done).
How much free disk space do you have?
Just some questions that might help us help you...
I don't know what size your hard disk is, but 48GB free isn't very much, and could possibly account for the sluggishness you're seeing. If I were you, I'd look at freeing up hard disk space so you have 100GB free. To do this, you can look at offloading your iTunes music collection, if it's large, to an external disk. Maybe even your photos if you have a lot of them. Definitely trash files you no longer need.
I see you're running Mountain Lion. I would also suggest booting from the recovery partition and running Disk Utility to Repair Disk, then Repair Permissions. After you've done that, reboot back to your normal ML partition.
Have you looked to see what applications are launching when your iMac is first booted up? Sometimes users don't know they've got apps running at bootup they don't need.
Hopefully, some of these suggestions will help, because you've got the computer power needed to run ML well.