@leinsters I have to admit, I don't think I've ever seen this situation of high altitude flights losing FPS. I've now seen three reports of that this week, which is making me think something, somewhere has a memory leak. I'm on a Mac, but have used Windows prior to this as well, and almost all of the flying I'll do is IFR with the IXEG 737.
I do wonder if X-Plane 11.5 has something up. Maybe even just Vulkan? I'm assuming you are using the Gizmo beta as your selection in the installer?
I'd be interested to know how your experience goes in OpenGL mode as well.
Most of what John and I speak of are more or less in relation to settings as a whole. But, departing somewhere and climbing out should be relatively uniform in FPS assuming your weather depicted is roughly the same.