On Sunday 10th March I updated xplane 11 (to version 188.8.131.52) and since then my fps is very low, barely 10 fps while it used to be 25-30. I tried changing the sliders in the graphics menu but could not get much improvement. I also tried the changes to the Xbox app, changes recommended on the xplane support site, but no improvement. It looks like the update changed something dramatically to the system. CPU usage is 45%, GPU 60% so both resources are not even fully used, which makes it even more difficult to understand.
The only change to my system was the update on Sunday, before that frame rates were OK.