Mac Pro (mid 2010) grinds to a halt when display is rotated to portrait
This is the sort of thing a well documented bug would let an engineer fix the code to avoid crippling a powerful Mac with a video change.


https://developer.apple.com/bugreporter/
Once you have the machine blocked, type sudo sysdiagnose in terminal and send that output in with the bug report as an attachment.
Usually, you never hear back, but sometimes you'll get an email requesting more details to help engineering isolate the behavior you are seeing.