I think Qt's is "better", in that GTK's just streams pixel data to a HTML canvas. Looks like Qt actually streams the WebGL drawing commands to the browser, which does the rendering itself. Certainly that's still not super fast, as the drawing commands are sent over a socket, vs. directly to the GPU's GL driver... but presumably more performant than rendering in the app (even if using the GPU to do so), and then copying the pixel data into network buffers.

Either way, it's a cool feature in both toolkits. (Not trying to belittle GTK here.)