@Holger,
My GUESS is a 1080ti with a 4k monitor. This video card isn't the best for this resolution, as you're already pushing things pretty hard without add-ons.
That said, let's take a look at things.
Can you post:
1. A screenshot of your FPS bar from X-Plane when the FPS are lowered.
2. A screenshot of you X-Plane rendering settings.