Hi Ekkehard,
The demo will perform exactly the same as the full sim, so if you are not satisfied with it, please do not purchase X-Plane 11.
In general, graphics will be limited by either the CPU or GPU (whichever is less powerful). If you haven't already, I recommend trying the process described in the manual to tune rendering settings to see if you can tune X-Plane 11 to a state you're happy with.