Keep in mind that X-Plane will be limited by either the CPU or GPU, depending on which one is less powerful. Please try following the instructions here on optimizing your PC's settings.
Keep in mind that changes in X-Plane are cumulative. If you add more buildings, shadows, clouds, etc, X-Plane now has to do more work in 360 degrees, so in general X-Plane will have lower frame rates than other games with a more tightly controlled (and limited) scenery environment.