Ejecta seems to use OpenGL to implement canvas. WebKit (thus Safari) does not do that, and there is significant performance gap there.

Company 100, Inc. maintains a patch to WebKit that use OpenGL to implement canvas. It is not yet merged, but I have tried it and it is significantly faster than stock WebKit for game scenario. So I have no trouble believing Ejecta is faster.

http://www.dorothybrowser.com/portfolio/canvasgl/