iOS UI thread blocked on initial load for large (dot)lottie animations (>5MB)

[majda107]

Hi lottie dev team,

when attempting to load large dotlottie animations (over 5 MB) that contain built-in assets, I experience brief UI thread blocking, typically lasting a few tenths to hundreds of milliseconds. This issue occurs only on the initial load, significantly impacting the user experience by momentarily freezing the interface.

After the initial load, animations appear cached, and the freezing issue no longer occurs.
I tried to solve this by creating a custom animation view controller with pre-loading functionality, but the issue still occurs on the first viewDidLoad event.

Switching rendering engine does not seem to have any effect on the issue.

Is there a way to load the animation without blocking main UI thread (but preferably showing a custom loader)?

Steps to Reproduce:

1. Use a (dot)lottie animation file larger than 5 MB with embedded assets.
2. Display the animation conditionally.
3. Observe the UI thread hang during the first render.

if showAnimation {                
   LottieView {
      await self.loadAnimation() // load larger animation with built in assets
   } placeholder: {
      ProgressView()
         .frame(width: 50, height: 50)
      }
      //.configuration(LottieConfiguration(renderingEngine: .mainThread))
      .logger(.printToConsole)
      .resizable()
}
            
Button("Toggle show animation") {
   showAnimation.toggle() // watch the UI thread hang after first click
}

Environment Details:

Platform: iOS
Lottie Version: 4.5.1, also tested on almost every 4.x.x version...