SwiftUI isn’t native and here’s why SwiftUI apps may sometimes feel janky. It ditches the key element that made animations run so smoothly on iOS since the original iPhone: Core Animation. In SwiftUI animations run in-app without CA backing. Block the main thread: SwiftUI ball stalls while UIKit’s keeps moving on the render server.
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You might be animating incorrectly in SwiftUI... nitter.cf/cifilter/status/210024…
Shannon Potter
@cifilter
Sep 16
Lot of speculation and misinformation floating around about SwiftUI today! I've done some reverse engineering of all relevant frameworks (including private frameworks), and I've made some interesting discoveries about SwiftUI that disprove a lot of the noise. More soon.
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It’s noting fancy, just a withAnimation call with repeat. Blocking is done with sleep. Happy to get corrected, but it’s confirmed as expected behavior (perhaps your block handler doesn’t run on the ui thread?)

Sep 16, 2026 · 4:16 PM UTC

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It definitely does. I assert a precondition of being on the main thread before sleeping. Happy to share my playground once I’ve ironed out a few more tests.
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Stripped down the code to minimum if you’d like to take a look (it uses scale here to save a few lines of code but the effect is the same) gist.github.com/kmagiera/349…

SwiftUI vs UIKit: scale animations during a main-thread block

SwiftUI vs UIKit: scale animations during a main-thread block - SwiftUIVsUIKitAnimations.swift

gist.github.com
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