I've already suggested an edit for a more accurate description.
It only happens when your project contains a custom font.
It'll freeze just by visiting any Storyboard/XIB that contains an IBDesignable custom view, not even referencing to that font in your Storyboard/XIB.
My workaround is to use have an older version of Xcode also installed (Xcode 6.3 beta or Xcode 6.2) and use that to update your Interface Builder files, and never open it in 6.3. open those Storyboard/XIB in a new window in Xcode with option
+shift
+click
the file in the project tree.
Whenever you encounter the freeze, I use this command to clear the saved state just for that project.
rm -rf YourProject.xcodeproj/project.xcworkspace/xcuserdata
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…