ADDITIONAL INFOS - READINGS FROM TASK MANAGER :
While XP11 shows the fps parameters like shown in my post above, the task manager shows the following values :
CPU : ~11% @ 1,2 GHz and ~ 1750 threads
RAM : 12,3/32 GB (38%) (in use 11.7 GB, avail : 19.9 GB)
Hard Disk : ~ 5 %
GPU : ~15 %
I think the pc is no longer fully powerfull as it was before the update. Thx Microsoft !!!
I have found the reason for my problem !! It was the Windows Defender - Antimalware Service Executable !!!
It ran a long time over the hard disc and checked X-Plane 11 everytime I pressed the .exe-file, too , until it has checked it completely. Thereafter XP11 started as fast as I know it.
Now I have XP11 excluded in the check process and everthing is fine.