The original (http://codepen.io/stuffit/pen/KrAwx) feels much more responsive (when feeding the same dimensions -- 114,53 -- as what your version uses), tested on Firefox 19.0.2 and Chrome 26.0.1410.65
EDIT: I shouldn't have said "no idea". There is one reason I can think of -- this uses WebGL, so if you have a bad card or drivers this might be slower. My algorithms might be more precise too, I don't know the other code.
EDIT2: A coworker just mentioned that I should turn off antialiasing (it's on by default) and that should help a lot of machines. Also optimized WebGL a few other ways.
I don't know what the cause is in my case, maybe WebGL on this thinkpad is really slower than CPU rendering on an Atom?