@Daniel - thanks for the tips, I'm actually using atf for one of my backgrounds (the quality of alpha rendering is fairly ho hum so am only able to use it for that) and am using the async property when creating the texture from it.
I'm using a reasonably complex mixture of a static TextureAtlas (a .png file) two DynamicTextureAtlases (so I can keep some of the rapid prototyping workflow I enjoy with the Flash IDE) coupled with Away3D (I've got a coverflow effect which isn't currently possibly in Starling... bring on the Sprite3D Class!
I started using Starling before the AssetManager was around, but am aware of the image decoding policy property you speak of. In my experience it's not the loading of the png into the app, but the upload and creation of the texture that freezes the UI and takes the time.
Obviously creating the TextureAtlas at runtime from emibap's class is causing a lot of the UI freeze upon app init, so I'd like to take a look at creating a pseudo threaded version of that class as well.
I'll report back with the speed difference in app initialisation when using two dynamic textureatlases compared to converting them to two atf files instead. I'd expect it to be a lot faster, but just how much will be interesting. The app currently takes just on 5 seconds to become "active".