JavaScript is not the same language as Java. They just share 4 letters of their name that’s all.
C++ and C# are also two complete different technologies.
And what your GPU does is more linked to the graphical engine than the language used. It’s just that with C++ they will be able to use OpenGL and DirectX to make rendering better with less resources (so actually your GTX780 will have something less to do)
What they have done for now with JS+WebGL is just amazing so i think we can trust them on quality of the work. That said i also can’t wait to see the native desktop client.
