I am also seeing this error, although it does not appear to impact my map rendering (I am not overlaying a route).

I gave up on SwiftUI previews as soon as I migrated my test project over to the main project for that very reason. I don't think it's my computer either because I bought a pretty high end MacBook Pro and they're still sluggish. I guess for prototyping you could always build them in a small project and port over, but that seems needless.

In my main project, I have ended up needing to use the method in the demo project: calling .onAppear and checking to see if the value matches the viewModel's value and updating the value as needed. It's a bit frustrating simply because one of the advantages of SwiftUI's declarative nature is that I should not need to manage the state in this way and it feels though adding the update value functions to each form item's subview is simply a hacky way to keep the view's data up-to-date.