I got into the same issue again updating to the latest version (11.34r I believe) last night : a significant drop in frame rate to < 20 fps. I did however notice that before the update Xplane would not open normally and I had to go via the task manager to "activate" it.
So I deleted xplane again and re-installed it, which gave me version 11.05. Got very good frame rates, also after installing plugins (air manager, pilotedge and spadnext). But soon it started going south again : Xplane not starting up correctly (need to go via the task manager) and more XSystem processes active than displays running. I upgraded to the latest version and of course the issue did not go away.
It looks like everytime I restart Xplane and resume the last flight another Xsystem process is added, further slowing down the system. It looks that the process servicing the main display is duplicated on every restart so I ended up having up to 4 processes running for the main display, one for the left display and one for the right display. This of course eats into the frame rate.
This gives me the impression that Xplane doesn't really quit correctly when I exit it. I did try to restart my computer but it didn't change. I will try starting a new flight again but based on yesterday night's experience I don't think it will help.
I attached the log file from my last flight :