I gather from the apple developer forums that this is an @IBDesignable issue. Especially in projects that use custom fonts, additional xibs, etc.
I have somehow fixed my issue by removing all @IBDesignable from swift UIView class definitions. You can open your project directory with TextMate or other, search and remove all “@IBDesignable”
However I still think this is a MAJOR bug, that needs to be worked on.. so keep filing bug reports to Apple.