I had the exact same problem: opening the storyboard in xcode results in beach ball with the Interface Builder task consuming all the memory. I got it back working by editing the xml of the storyboard file and setting
useAutolayout="NO"
in the document node. Of cause this is not a solution to the problem (which seems to be a bug in xcode) because it screws all constraints, but at least I could open and compile the storyboard again.