You can check using the network tab in the webkit inspector, or firebug. I don't see any async loading.
The html pages have this header:
Transfer-Encoding:chunked
This means the html is streamed. The response on the sever-side is near-instant, only being hit for DNS the first time, and just the round trip latency the second. Due to the streaming, and the CSS being included at the top, the css is loaded super fast, in parallel with the html. In less than 100ms (for me), most things required to render the page are loaded up.
The problem is that for games--for smooth games with low invocation overhead--we reallyreally need to be able to use the graphics card. There are cute tricks you can do with a 2D context (including, in Three.js's case, software fallbacks for some of WebGL), but nothing that really compares to being able to write modern programmable shading pipeline code.
Without that (and the Audio API, and Orientation events), game developers don't really care about your platform.
Now, there is (in my opinion) a storm brewing for WebGL. Go here ( http://www.khronos.org/registry/webgl/extensions/ ) to see the coming madness. There is nothing worse in the world than being under the stewardship of Khronos/ARB. The fact that W3C et al. haven't taken over or provided an alternative to this is bad.
Microsoft could do something like make DirectWebX or Web3D or what have you. That would be annoying, but sure--we've been writing cross-API rendering backends for a decade and a half now, and so this wouldn't be terrible. Even better, Microsoft could go ahead with a WebGL implementation that calls down into DirectX.
Best, Microsoft could leverage their experience with good API design--don't scoff, seriously; compare OpenGL and the latest DirectX versions--and work with the W3C and browser vendors to fix some of the yawning holes in WebGL.
(
For example, killing off the extensions mechanism entirely in favor of "Is this Web3D 10 compliant? If so, you get this and don't worry.", or a useful capabilities structure. getParameter() is kind of useful, but having a guaranteed caps baseline is nice.
)
It may be smooth, but my job is to make cross-browser HTML5 games (a great gig if you can get it) and almost all of my worries are 'solved in Chrome and Firefox' but that fix somehow is not available in IE9.
IE10 might, be better but then you have to deal with all the legacy people who don't upgrade. Also when it ships, it will still be behind Firefox and Chrome.
Does it use something like this? http://alexmaccaw.co.uk/posts/async_ui