Ok, in this case, your computer must be not so bad.. if I were you, if I wanted to perform tests where time matters, i would set every graphic settings at minimum, and fly over the ocean, with as little land land as possible. No cloud.
I would also delete .../x-plane 11/output/Preferences folder. You would lose all your settings but this very often improves fps.
What are you computer specifications? Can you attach you .../X-Plane 11/log.txt?