X-Plane is the main issue here. Although it seems obvious that a more powerful graphics card would be able to generate the frames quicker, so if I want to generate two times the number of frames when my GPU is idling most of the time then it should have plenty of capacity, this is only a very small part of the story.
In reality, the GPU still needs to be told by the CPU what to draw and where. For every drawing loop, the CPU needs to tell the graphics engine where every object in the scene needs drawn. This is why you get such a hit when going to VR. Were effectively attempting to draw each scene twice and were having to wait on the CPU to do it, even though the GPU could probably handle it fine. Also VR was added to the existing framework rather than the framework created for VR - there are better ways of adding stereoscopic rendering to the render pipeline but these certainly wouldn’t be possible with the engine X-Plane has.
This is compounded by the fact that X-Plane’s game engine does almost everything in the same single thread.
Part of the problem is that the graphics engine is based on OpenGL which is then converted to something the graphics cards can actually process.
The good news is that X-Plane is moving to the much more modern Vulcan/Metal (for Mac) graphics engine which is a lot more efficient - the same scene can be rendered with about 50% of the function calls compared with OpenGL. (https://developer.x-plane.com/2019/12/vulkan-and-metal-testing-and-bug-fighting/ and https://developer.x-plane.com/2019/12/the-vulkan-metal-public-beta-will-be-in-2020/)
The second part of good news is that the move to the new engine also facilitates the possibility to move some of the other parts of the x-plane engine to seperate threads, which would also help.
One downside of this for nVidia owners is that , from the testing they did earlier in the year, the increase in performance just from the switch to Vulkan was about 5-10%. For AMD owners it was more like 40% (from memory). This suggests that the nVidia were actually really well optimized for processing OpenGL requests and the AMD ones were not so well optimized.
Still - an increase of 5-10% is still good, and thats without moving any of the other processes to other threads, and maybe they have been able to optimized further since then. We should know soon because it looks like a public beta some time soon(ish).
As for the future - we shall see what MSFS2020 has in store. Although it doesn’t look like they will support VR at launch (although they keep surprising us with new announcements) this is practically a new engine (very loosely based on FSX but sounds like large chunks have been rewritten) and its coming from a Asobo studio that have a history with working on AR/VR. Originally VR wasn’t really on their radar but since they invited a number of people from youtube/flight sim community to a preview event back in September it was increased in their priority list because most people were requesting it.
2020 (the year) is going to be exciting for taking to the skies…