Shelxtl XP, written by George Sheldrick, used to work very well under OSX 10.4. With the X11 that comes with OSX 10.5 and later, however, performance is poor. PROJ rotation of small structures is painfully slow, but for larger structures or extended atom lists, such as what you get after GROW or MGEN, rotation can be fine. The official explanation is that the slow down is caused by "many layers of inefficient code". However, large atom lists seem to rotate quite well with PROJ, so something else is going on. If that were the reason, then why did it work so well under OS X 10.4, and why would the performance bog down be worse for small atom lists? Perhaps a fresh compile would fix it, but the source code is not available. I had a fix for this issue on OSX 10.5 and 10.6, but that solution does not work on OSX 10.9 so I had to find a different workaround.