Out of curiosity - on my machine, a GTX1080 with 16GB RAM, I can see the FPS drop from 60 outside to 30-35 inside the F-15E, but I barely notice it, things remain silky smooth as it’s faster still in frames than the human eye can detect. This remains the same in VR ( Oculus Rift ).
What is it that happens on your machine that makes it so slow? Are your settings too high perhaps or something? Just interested to find out why some people have no issues at all with the F-15E / I while others do.