In terms of CPU usage, the CPU in DCS should be mostly calculating the flight dynamics, collision detection and general game logic.
Yes certain areas of the map are full of buildings and other objects, but most of them are just simple shapes with virtually no modern day post-processing effects. DCS still uses antiquated. antialiasing methods. No FXAA which is much more efficient.
If theres any slow down on a decent gaming rig I would put it down to the engine struggling rather than anything else.