Shelxtl XP, written by George Sheldrick, used to work very well under OSX 10.4. With the X11 version that comes with OSX 10.5 and 10.6, however, performance is unpredictable at best. With PROJ, rotation of small structures is painfully slow, but for larger structures and 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", but because large atom lists seem to rotate quite well in PROJ, I suspect the official diagnosis is not right. If that were the reason, then why did it work so well under OS X 10.4, and why does performance bog down only for small atom lists ? I've also noticed that PROJ rotation on my newer 64 bit linux boxes can be choppy and unpredictable. Maybe it's a 64bit vs 32bit issue, or a race condition or similar nonsense. Either way, I bet a fresh compile would solve the problem, but there's no way to tell because the source code is not available.
Note: This fix works up until OSX 10.6.8. It does not work on OSX 10.9 "Mavericks",
and probably doesn't work on either 10.7.x or 10.8.x (I haven't tried). I have
a new, simple workaround that does not require installation of an older
version of X11, and does work with OSX 10.9.
Instructions are given here.
This workaround installs the version of X11 from OSX 10.4. The way the older and
newer X11 versions are organized in OSX, it is possible to have them running
concurrently. The process is straightforward enough, and it is definitely worth doing.
On my flaptop (15" 2.66GHz i7, 8GB, OSX 10.6.8), I have both versions of X11
installed, and they run fine simultaneously. Here's a list of instructions with some
explanatory notes. It looks like a lot of steps, but really it isn't. It assumes that
you will be downloading the needed files to the Desktop. This is just for convenience,
it doesn't matter where you put them.