2012 base model Imac with 16 gb of ram. Not immediately after upgrading to El Capitan, but shortly thereafter, I started noticing tons of lag. It got worse to the point that all I was looking at was the beach ball spinning, and programs would take ages to respond to anything. I looked at the activity monitor and my processor was not being stressed, and I was not even using 50% of the ram. I ran the diagnostic on the hard drive, and it came back ok. I tried systematically force quitting programs, and it didn't help. Then I went nuclear option, and reinstalled the whole thing from a fresh install. Did not go from time machine, and systematically installed all the dmgs from my programs. It is a bit better now but no where near as snappy. I still get beach balls, and in activity monitor I am showing even less strain. The HD is 80% empty. I am out of ideas here. Anything else I should try before trudging this heavy ass computer to the Apple Store?