Hah I have the same question! I've been working on Lottie for over a year, but I've never used After Effects before and don't have it installed. My life would sure be a bit easier if there was a nice lightweight tool for creating and modifying Lottie animations. One useful tool is the lottiefiles.com editor, but its really barebones and only lets you modify colors of layers in existing animations.
Core Animation is designed to move work related to playing animations off of the app's main thread. Animations are instead performed by a different process, the OS render server. The UIViews / CALayers / CAAnimations themselves have to be configured on the main thread, but the work to actually play the animation and render each individual frame doesn't happen on the main thread.
