Closing the old project seems to bring back responsiveness. Last time I did that, Xcode error alert told me the storyboard couldn't be saved, though I hadn't made any changes to it.

I do need the old project - the new project is a rewrite and update of it - but I don't need to open IB and can just view the old code files without Xcode. 

I will reproduce and submit a report via Feedback Assistant.

Best

Phil