Also, how many applications do you run in the background simultaneously while working with another application?
How full is your iMac's internal hard drive?
It really sounds like you simply do not have enough RAM in your iMac to do the tasks you want to do since upgrading to OS X 10.8 Mounain Lion.
Both OS X 10.7 Lion and OS X 10.8 Mountain Lion use a lot more CPU, GPU, RAM and hard drive resource than previous versions of OS X.
Did the slow down occur after the upgrade to OS X 10.8?
If it did, then it is because OS X now needs between 2-4 GBs of RAM alone for the OS to run smoothly and efficiently.Once you start using other apps alongside OS X, both OS X and the App/s you are using increase the demand for physical RAM. Without enough RAM, RAM intensive task get sent to the iMac's internal hard drive to process tasks. A spinning hard drive is much, much slower to process tasks than actual physical RAM and slowing down your iMac is the result of this.
Your model iMac can take up to 16 GBs of RAM.
My advice, install, at least, another 4 GBs of RAM to bring your total RAM to 8 GBs.
If you can afford to install more RAM...do that!
Purchase correct and reliable Mac RAM from Mac RAM sources Crucial memory or OWC (macsales.com).
Good Luck!