Thanks for the feedback! That is definitely a nice feature to have.
In the meantime (disclaimer: I haven't tried it myself), if you are willing to edit the exported code, you could potentially start the Animation earlier by editing the start listener to listen to the "DOMContentLoaded" instead of the "load" e.g.
and then using the first scene as a loading screen. However, if you'd want to automatically continue on when the loading was done, you'd have to add another listener for the "load" event or continuously check the page status and react on that as well.