There’s the overhead of pushing updates to the UI layer from your app code, having it trigger an update, which in turn makes it check what other things have updated and what needs to redraw. UIKit is handling all that for you, but as a result you’re not getting full control of when you’re updating. You get pacing issues, where you can’t really guarantee you’re drawing at set intervals either which can lead to pacing stutters since you’re beholden to the apps event loop.

With an immediate mode UI in a game, yes there’s more cost to set up your draws, but you have more direct control of when everything will update because you’re handling the event loop more directly.

As for other priorities, I don’t mean just app priority. I mean intra-app priority. Lottie has to mix in with user code, the app it’s within might schedule a task with a higher QoS priority. It’s not just showing a fixed animation, it’s part of the app interaction model. So it’s easy to get into a situation where the rest of the app introduces stutters inadvertently.

Using CoreAnimation lets you sort of side step that discussion.

And yes I agree games have it harder over all because they have to handle everything. That wasn’t my point really. I think though that games have it easier when it comes to frame timing as a result of being able to control the entire event loop. So it’s easier for a game to reason about and guarantee smoothness of motion than it is for a framework that has to fit into arbitrary applications.