RLGaming.com

Supposedly when you choose Low/Med/High settings there are some 'under the hood' changes that go into affect along with the visible drop downs. Select the option that most closely reflects the settings you want and tweak the few settings you want to change from there. Don't, for instance choose low, and turn everything up to high afterwards. I was a bit dubious of this, but after choosing 'High' and just leaving it, I get much higher performance.

Also, Civilian Traffic is apparently a huge CPU hog, so best to leave it off. I will be forcing this off in the missions I created to help with performance, as I believe it is on and probably murdering our patience during play.

I noticed one of the missions we played had tons of civilian cars and buses on the roads so eliminating that should help out quite a bit.

Ya, I didn't know it was so hard on the CPU; I thought it just added atmosphere, so I cranked it up... bad move on my part...