My MacBook Pro is a Late 2011, and the single best thing I did to it is put a Solid State drive in it. Extra RAM is also worth doing, but if you’ve only got the cash for one thing then do the drive, it’s like night and day!
Not sure how easy this is on an iMac though, as they can be tricky to get into and even trickier to put back together!
Routine cleaning is recommended, sometimes even a full, clean install of your OS from time to time.
also, which OS are you running?