As someone who grew up in a time where you had to tweak your config.sys and autoexec.bat files so that you could get a few extra FPS on Duke Nukem 3D, I can tell you: this is pretty freaking amazing.
>One concern for this type of product is that there's no concept of "installing" a webapp. The website will need to stream all game resources to the client for each user. I would wager that's one reason the graphics are poor: all of what you see was probably generated from <100MB of content.
As the demo says, it's recommended to try it in Firefox Nightly. The demo starts up and runs much faster there thanks to optimizations that are not yet in the stable release (which is what you're running, I think).
