Check your GPU driver

[Jakob Ludwig]

I found out that the newest drivers from nvidia, let the FPS drop dramatically with my GTX 660Ti. So I went back to 320.49 version, which I had before and everything is working like a charm again.

 

So if you experience a very poor performance with Skymaxx Pro, consider to try an older driver for your GPU. Looks like that the latest, are not superb for X-Plane... 

[sqrt(-1)]

I haven't noticed any reduction of performance with 331.82 on my GTX 680.