I am facing exactly the same issue Xcode 12.3 is not Responding after a few seconds I open SwiftUI project. I have extremely annoying problem with autocompletion too under SwiftUI project -> there is no autocompletion especially in viewModel files (I have to wait ~30sek to see somethig). In Xcode 12.3 I am not be able to develop my apps. Apple please do something with those issues!