I can confirm the tree rendering bug and the stutters.
It happened to me only once, and then after something like 10 to 15 seconds went back to normal. I was doing the first mission of Fortress Mozdok campaign in the Su-27, just leaving the parking area towards runway 8. FPS was like 0.5.
System: GTX 970, Core i5 4670, 8GB RAM (with the current leaks, I can fly one mission tops, if I'm quick)