Listen to me, as if I know what I'm talking about...! C2 is unable to set its own internal clock, so game mechanics don't update if the browser can't be bothered to redraw the canvas for whatever reason Google thinks is acceptable. The weakness with c2 lies here and only here because many behaviours like physics fail when dt momentarily doubles.
No one would notice an occasional dropped frame if the next one was correctly drawn with all objects updated as if they moved smoothly - a momentary drop to 30 fps would be transparent to the player if this was / could be implemented. In my humble opinion...