The campaign is fairly heavy in performance demand as it's very detailed, but having said that it sounds like you're having a bigger impact than most, most issues I'm aware of (pre multi threading) were in VR.
Few things worth mentioning to check and be aware of. Incirlik, especially the shelters, are not a great area for performance...that's just a Syria map thing. Once away from the base things settle down a lot more.
Hopefully you're running the multi threading version, if not I would advise to do so as the high unit counts in the campaign benefit from this.
If you use TACVIEW then try disabling it, TACVIEW takes CPU resources and this is made worse with the amount of things it has to track in the campaign.
Finally if you have any Win-wing devices then disable the scripts they use in DCS as these are real performance hogs.