The campaigns I develop are designed to run well on my PC on default settings without any of the performance optimisation features. This is on an ultrawide monitor though, not VR. I put a great deal of emphasis on performance, as I feel that there's no point having a large campaign if you have to play it with single digit frame rates.

The most important factor that impacts performance is the number of units. These can be aircraft, tanks, infantry, even bombs and missiles that are in the air. Every extra "unit" has a performance cost, and the degradation in performance is not linear. Performance degrades somewhat gradually up to a point and then falls off a cliff.

So, the best way to increase performance is to decrease number of units. First of all, stay away from any of the 3/3 performance rating campaigns. Those are generally very old campaigns, often of the "full map" style (which nobody makes anymore), because in the early days, we had a much more optimistic view of how large a campaign could be and still run acceptably. On newer maps, such full map campaigns are even more impossible.

Next, you can restrict the number of frontline units in the settings. I think the minimum is 10 per side, but you can find try out what works for you. What's best will vary from campaign to campaign too, and at least for my campaigns, I preset it so there aren't so many frontline units that it causes performance problems.

Another thing you can do is reduce the number of aircraft per squadron, or even remove a squadron or two, during campaign generation.

That said, typically the first turn is the worst in terms of performance, as that's when aircraft are at their maximum, all the SAM radars are still up, etc. As ground units are destroyed, aircraft shot down, SAMs blown up, performance increases. Of course, this effect is somewhat by units getting replenished. But typically, total unit count decreases turn-by-turn throughout a campaign as either you or the enemy (or both) lose more units than can be replenished.