Over the last few weeks, I've been messing around with my PC trying to get Squawkbox 3 to work. I've adjusted basically every setting I could think of with no success, and then put all settings back to where they were, or so I thought. Over the last week, I've been getting framerates in the 8 - 14 range and was getting very frustrated. Then I finally remembered I had bumped my monitor refresh rate up to 75 Hertz. I changed it back to 70 Hertz, and am now getting 20-25 FPS flying complex add-on aircraft again! WOOHOO!!
So if you've exhausted every other avenue in search of better frame rates, see if it's your refresh rate.