I remember the experience of Unreal Engine demos compiled to asmjs a decade ago being more or less the same, taking absolutely forever to load before anything happens. It doesn't feel like much progress has been made on that front.
Those aren't as bad, about 20 seconds to get going from a cold cache. Car Configurator is incredibly slow though, I timed it at about 8 minutes to load from scratch and when it finally got there it was unusably laggy until I refreshed and let it start again with a pre-warmed cache. According to Chromes internal task manager the CarConfig tab is continuously using about 500% CPU (i.e. five entire cores) and 5GB of RAM despite very little going on in the demo. This is on a current-gen desktop processor.
That's pretty neat, not much in the way of progress bar for me but they load in about 30 seconds. And it seems a little choppy at times but playable. Reminds me of late stage flash era games.