Suggestion: Start with the online manual section Configuring and Tuning Your X-Plane Installation
Hint - Settings:
Visual Effects: Minimal (Important!)
Texture Quality: Medium
Antialiasing: 1x (Very Important!)
Number of World obj: Low
Reflection Detail: Minimal (Very Important!)
Draw Parked: checked
Hint - Flight Configuration -> AI Aircraft: Delete all
Check flying, check approach to a small airport, check approach to a major airport.
Then if want optimum flying: Download Flying_with_Lua plug-in, and the FPSWizard11 script. This thing is a miracle that allows me to fly into major airports and maintain real-time (over 19.9 fps) simulation.