Very slow on Firefox (both wrt frame rate and actual game speed) and gets down to single digits FPS after a while.

But the slow speed and weak acceleration (did you forget that v = v + a*dt and not just v = v + a? :)) feels actually nice and meditative, if only the FPS were smoother. Tried it on Chrome and it's a much more dynamic and fast-paced experience on it.

(Edit: took a look at the README, loved it!)