I have tried the demo several times now and I can not have it run smoothly on my mac. It runs slow, and there is a lot of frame clipping/missing, although I see that the framerate is around 40 fps.
I have also tried to do some adjustments to the rendering options. Do you have any clues?
I am running X-Plane 64 bit on this computer:
Macpro (mid2012)
OSX Yosemite 10.10.2
2X2,66 GHz 6-core
24 GB 1333 MHz DDR3 ECC
ATI Radeon HD 5870 1024 MB