All I have is my own experience. In my current app, my views are 100% SwiftUI. One of them uses a SwiftUI List which typically has hundreds of thousands of items, each with title, subtitle, duration, cover art, etc. I expect it would still be butter-smooth at 1M items.

Was my initial naive implementation as performant? No. My point is that optimization still matters in SwiftUI, and that optimization looks quite a bit different in SwiftUI than it does in AppKit/UIKit.