That worked nicely even though Canvas is not being accelerated by 3D hardware. CPU consumption was noticeable but still really low (I have a decent CPU though, must try on netbook).
The thing that Rocket Engine promises is NOT Canvas. It's an HTML DOM engine, and is this DOM manipulation (traversal, insertion, deletion) which is slow, due to the standard not being geared to speed. Or ease of use. Or being just nice. Ugh.
Having seen this demo, though, and knowing IE9 will feature hardware acceleration for 2D (which means others will probably follow) makes me long for the arrival of framerate-independent 2D games for HTML5-Canvas. I wonder if there are any stretching options
Edit: Acceleration goodness in IE9