OP - I've been working with the Game Closure team on the HTML5 devkit for quite a while now, and we have yet to find an engine that gets better performance out of 2D JS games on native. The view hierarchy, rendering, and animation are written in both JS (web builds) and C (native builds), connected by a Java Android stack and an Objective-C iOS stack. The docs are a bit out-of-date but the tech is really powerful. It supports WebGL and canvas rendering in web builds, but there's no 3D support yet.

Github: https://github.com/gameclosure/devkit Docs: http://docs.gameclosure.com/