ThreeJS is a library on top of WebGL. It's an implementation detail -- whether it uses ThreeJS, stackgl, or your home-brew library, it still uses WebGL as its primary technology.
Maybe you should read my comment again? It's that ThreeJS abstraction library that's slow and why it's not okay to say it's a WebGL demo, it's a Three.js demo. As you can see with comments above people blame it to WebGL is slow - which is simply incorrect as WebGL is based on OpenGL ES 2.0 and exposed as HTML5 API. If you hand tune the JS and shader code WebGL can be very fast and short startup time. You won't get that benefits from generic fat libraries.
Well, by that logic, everything is written in assembly since everything uses assembly as its primary technology.
RISC or CISC?
Though you may have read the title as "game written in webgl" I'm pretty sure it's meant to be read as "game using webgl". My guess is you and frik read it using the former interpretation while others used the latter.