An update on my "journey" :
- I logged the bug report and xplane support asked to run a standard frame rate test (with a B777 I believe) and surprisingly I got consistently above 20 fps which made me believe this was due to the fact that this test only ran on one screen (vs the 3 screens I use). According to xplane support the test proves nothing's wrong with xplane ... . They did not reply to further questions from my side.
- I tried running the simulator again with the C172 (the plane I always use) but still frame rates were below expectations, about 15-16 fps. Even if I switched off two screens it did not help.
- I tried most of the tricks you can find on Internet (like changing CPU minimum power management, deleting preferences, ...) but no improvement. The frame rate seems to be "stuck". Also disabling plugins does not change anything.
- I downloaded MSI afterburner to allow me to get a real time view on the use of hardware resources while I run xplane, however the framerate displayed by MSI afterburner is much higher than the one xplane 11 displays which is strange. Based on how slow the simulation reacts I must believe the xplane 11 number is correct and not the number from MSI afterburner. Using MSI afterburner I got confirmation that CPU and GPU load is not the issue (30 and 50% respectively) but GPU memory is fully used and sometimes I use the full 16 GB ram, which is surprising (never happened before based on periodic checking in the task manager).
- I tried putting all graphics sliders to the absolute minimum but this did not give any improvement in frame rate. It looks almost like something is blocking the frame rate.
- Strange thing I noticed : when starting up xplane 11 nothing happens. I have to go the task manager to activate the process, otherwise I just get a black screen. This has been like that for some time but it never gave issues before so I didn't worry. Now, with these fps issues, I start finding it strange. Another weird thing : in the task manager I have 7 XSystem processes being mentioned under Xplane 11. This seems strange. However as soon as I stop one process Xplane 11 shuts down. Not sure if this is related to the fps problem but it's strange.
In a nutshell : my simulator has been out of use for 3 weeks now thanks to this update ...