Looking at chrome's task manager, after just loading the page it's memory use grows from 60MB to 260MB before falling back to 60MB, repeating every 4-5 seconds. That seems a bit wasteful.

processing.js is hardly indicative of HTML5 game performance in general. Also, if this code were to actually only redraw what needs to be redrawn (the paddles and ball), CPU usage would be near 0.