Same here, on a long file it can go for 30-40 mins and then eventually Xcode crashes.

There seems to be a bug in the latest Xcode or macOS causing this. I have storyboards with custom fonts that work fine, but whenever I go to the font menu, it won't show any custom fonts. I haven't changed anything in my app, so it's likely a bug that Apple needs to fix. I suspect you can manually edit the storyboard XML files to add the custom font names, but that's not a simple task.