Just tried it out and sharing my experiences. It is quite choppy for me, only getting a few frames per second, on 2K monitor, but I'm on a computer that will happily play Civ 6 on low quality. I wasn't expecting it to be fast, though.
Highest Gfx settings (default). 2800x1700px window. Best out of 2 runs:
Chrome 56: 33 fps
Safari 10.0.3: 29 fps
Firefox 51: 13 fps
I can still play Team Fortress 2 at 1080p just fine on this tho ;)
That's somewhat disappointing, both in absolute terms as well as compared to your values. Although maybe it's being throttled at around 40fps which is probably beyond the point of diminishing returns?
Edit: Getting 35fps in Firefox, which seems to support the idea that it's being throttled somewhere around that value.
EDIT: With low settins & AA off, I get 12 fps.
Miserable :(
FPS: 1. Chrome Version 55.0.2883.87 m (64-bit) on Windows 10. AMD A8-3850 APU with Radeon HD Graphics 2.90 GHz 16 GB ram
On Firefox 51, I get 2 FPS (although probably roundoff error).
Doesn't matter if I use low textures without anti-aliasing.
I should not the CPU usage for shoots way up. So most likely the WebGL is not utilizing the GPU. I should note that the GPU is integrated into the CPU chip as an "APU"...maybe this particular model of APU isn't supported by the WebGL.