There's always been a lot of room for them to optimise performance on macOS to a level that they do iOS, but modern Apple basically wants macOS to go away and sees it as more of an annoyance, so they don't spend any time tuning it. Then the elephant in the room is SwiftUI itself, there's a performance cliff there, wherein if you've optimised everything else, you might just hit the brick wall that's the layout engine with no recourse or ability to even peak under the hood. We're at a point now where building a fully native macOS app, with the first-party toolchain, will give you far worse performance (in terms of responsiveness not resource usage) than something like Electron. I have a suspicion teams inside of Apple are also running up against these issues as they start to actually adopt the framework.