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.)
AFAIU for 'heavy' 3D applications like AAA games the BW over the PCIe bus to the GPU can be a lot higher than what comes out of the cable to the screen. So just sending (compressed) pixels is not necessarily a bad remoting strategy.
Though for something simple like just rendering GUI widgets the opposite may very well be true.
https://doc.qt.io/qt-5/webgl.html
I've used this to let friends on IRC paint on my LED shelf (https://github.com/eikehein/hyelicht), which has a Qt-based embedded and phone app GUI, over the internet. Cheap fun!