I just recently downloaded the latest public release of Xcode (6.3) and I noticed the storyboard for my app is now low res on my Retina MacBook Pro. Images, text and standard UI
I submitted this as a bug report, and Apple responded by saying that it is the desired appearance. They did not give an explanation, that was all they said.
Have you tried re-installation of the same version? That helped once. Alternatively, can upgrade to newer version, too.
It appears to be related to a bug in the newest iOS SDK, and as a workaround, they've forced Interface Builder to render everything at 1x. Currently, there doesn't seem to be any solution to this annoying problem other than to wait.
Just installed Xcode version 6.3.2 and this issue appears to have been fixed.