I viewed three examples simultaneously from the Phosphor gallery at http://www.divergentmedia.com/phosphor/gallery and each one only used about 5% CPU, for 15% total. I closed them and viewed a YouTube HTML5 video (no Flash on this machine) and it consistently used 70% CPU. It seems the Phosphor approach is more lightweight than video and it was entirely smooth (on a 2007 dual core MacBook Pro running a 64-bit Linux). If you don't need audio and want to include a video or presentation in your page that doesn't require a plugin and looks to be supported on a wide range of browsers and platforms, this looks like a compelling option.