Another little report:
There can be quite a time lag after the scene is 100% Loaded (for more complex scenes). I found, the more lights and shadows in the scene the more lagging. Do the generated console warnings have anything to do with that? I have included some screenshots.
Yes, currently the preloader only track the loading of geometry (gltf + bin files). In most cases its ok, but if you got a lot of big textures and numerous materials it will take extra time to process your these images and compile shaders and this is why you see this pause.
Still I agree that something is needed to be done in this field. Thanks for your feedback!