I can confirm that this bug exists in Xcode 12.5/iOS 14 and also the latest beta of Xcode 13/iOS 15.

So initially, I didn't see any improvement in Xcode 12.5.1 - notably the iPhone 12 Pro simulator was still hanging for around 15 seconds before my app UI became responsive. However, erasing all contents and settings in that simulator seems to have restored performance to normal. I thought I'd tried this before so maybe something has improved.

For the record these additional keys are: NSAllowsArbitraryLoadsInWebContentUsageDescription NSATSExceptionUsage NSATSExceptionUsageUserChosenContent