Shelxtl XP, written by George Sheldrick, used to work very well on 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 due to "many layers of inefficient code". However, large atom lists rotate quite well with PROJ, so something else is going on. If that were the reason, then why did it work so well on OSX 10.4 and why would performance be worse for small atom lists? Maybe a fresh compile would help, but the source code is not available. I had a fix for this problem on OSX 10.5 and 10.6, but that solution does not work on OSX 10.9 (or later) so I had to find a different workaround.