Spotlight could be indexing your hard drive?
Check the top upper right in the OS X main menu bar, look for the magnifying glass symbol and see if there is a dot in the middle of the symbol indicating that the Spotlight function is indexing your iMac's hard drive.
Do you run any antivirus software on your Mac? Antivirus software can slow down the normal operation of OS X.
Do you run any "crapware" like Mackeeper or any other type of "crapware" like so called hard drive "cleaning" apps?
How many applications do you run simultaneously in the background while working in another application?
Try and cut down on the number of simultaneously running apps.