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I have a similar problem (until I found this question I felt like the only SwiftUI dev targeting macOS…). I think it's a SwiftUI bug. I have the Table backed by FetchedResults, and the performance issue also happens when I change the sortDescriptors.

Anyway, I am using an ugly workaround where I set a tableIsRefreshing state variable to true and replace the Table with a ProgressIndicator meanwhile. Adapted to your code this might look like this:

struct TableTest: View {

    @State private var tableIsRefreshing = false
    @State private var items: [Person] = (0..<500000).map { _ in Person() }

    var body: some View {
        VStack {
            if tableIsRefreshing {
                VStack {
                    Spacer()
                    ProgressView()
                    Spacer()
                }
            } else {
                Table(items) {
                    TableColumn("Name", value: \.name)
                    TableColumn("Age") { person in Text(String(person.age)) }
                }
            }
        }
        .toolbar {
            ToolbarItem() {
                Button("Clear") {
                    tableIsRefreshing = true
                    items = []
                    tableIsRefreshing = false
                }
            }
        }
    }

    struct Person: Identifiable {

        let id = UUID()
        let name: String = ["Olivia", "Elijah", "Leilani", "Gabriel", "Eleanor", "Sebastian", "Zoey", "Muhammad"].randomElement()!
        let age: Int = Int.random(in: 18...110)
    }
}

You might need to add a DispatchQueue.main.asyncAfter delay if this doesn't work.

I hope the bug gets fixed, please submit a bug report similar to mine: http://www.openradar.me/FB13639482

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