It comes down to understanding the reactivity model - how to define state that can change, and how efficiently the UI updates in response to those state changes.

I think the issue the author had with SwiftUI is not understanding the reactivity system, or frequently choosing an expedient option that caused over-rendering - unnecessary re-renders in response to state changes that aren't required for the UX to work correctly from the user's perspective. Even if your UI's actual rendering work takes 1ms to paint, you want to avoid re-calculating busy work, allocation, etc, which can drive poor experience.

With a framework like React that provides very few primitives, it's quite clear when the framework will re-render. With a complex framework like SwiftUI, there are many primitives and different ways to represent state, subscribe to updates, etc, and the framework is quite under-documented. After spending 15 hours with SwiftUI, I still don't have a clear understanding of how the system detects a subscription, or what the re-render semantics are.

React I would describe this way:

1. Every re-render starts when you setState to a value that does not (!==) equal the current value in a parent component, or a reducer stores a new value internally.

2. Child components of that component will re-render except in two cases:

2a. The child component uses React.memo, and the props passed by the parent are shallow-equal to the previous props

2b. The child component JSX nodes are strict-equal to those returned by the previous render of the parent component, eg a subtree in the parent wrapped in useMemo

I can't come up with a similar definition for SwiftUI after 15 hours building an app in it. I need to consider @Binding, @State, ObservableObject, @ObservedObject, projected values. When is the system creating fine-grained subscriptions automatically? When is the system subscribing to an entire object (deeply?). Hard to find a cogent explanation of all the options.

Because of my different understanding of these frameworks, I personally write React apps that perform better than most people expect web apps to perform, and SwiftUI apps that perform much worse than people expect Apple native apps to perform.