I had a very similar issue after upgrading to macOS 27.0 / Xcode 27.0 on an Apple Silicon Mac. Opening Storyboards/XIBs made Xcode extremely slow or completely unresponsive. I was also able to reproduce it with a brand-new Xcode project, so it wasn't related to my project's dependencies or code. What eventually made Xcode usable again was resetting Interface Builder's simulator data and reinstalling the iOS 27 Simulator Runtime. I closed Xcode and moved the Interface Builder support directory: ~/Library/Developer/Xcode/UserData/IB Support to a backup location, then removed and reinstalled the iOS 27 Simulator Runtime from Xcode Settings → Components. After that, the first Storyboard load was still somewhat slow (~20 seconds), and Interface Builder initially used quite a lot of CPU while its private simulators initialized, but after that XIBs/Storyboards became usable again. I've since rebooted the Mac several times and tested multiple projects, Storyboards and XIBs, and the severe hangs haven't returned. I would keep a backup of the "IB Support" directory instead of deleting it initially.

Same issue here!