What hardware does Xcode storyboard performance depend on the most?
I had the same issue. Whenever I opened the storyboard after the inconsistencies message was displayed, Xcode would consume 60%+ CPU and essentially make it unusable.

I (very slowly) found which images were being reported as problems, then deleted them through finder and restarted Xcode. You may also need to remove any references to these images in the storyboard.

Xcode now runs fine - hope it helps!