My iMac is running very slow, please help!

My iMac is a little old for computers. It is running VERY slow lately. I do a lot of video editing for work among other things and it can't handle more than one or two things running at once without the spinning wheel coming up. I honestly click on something and the wheel spins for 5 seconds on average then I can do the next thing and it continues. It has progressively been getting worse and is to a point that I am probably going to buy a new iMac with upgrades. Here is the info. Any help is greatly appreciated.

iMac 27 Inch

Mid-2010

2.8 Gz Intel Core i5

12 GB 1333 MHz DDR3 Memory

Storage: 493.28 GB free of 999.35 GB

Anti-Virus - Kaspersky (have had this since purchase with updates obviously

Not sure what else anyone would need. I have emptied trash in iPhoto, iMovie, and everything else. I have run Onyx safely for years. Am I just trying to do too much at once with a slow processor and low memory? Thanks!