HTML5 playback is getting better but still a pig compared to other methods.
360P Youtube in TFF with Bluhell & NoScript on my 2.1 G5 iMac:
HTML5 - 90-95% CPU
Greasemonkey/Viewtube/VLC - 22% CPU (with TFF minimised, 46 when not)
Greasemonkey/Viewtube/Coreplayer - 20% CPU (with TFF minimised, 45 when not)
Greasemonkey/Viewtube/Mplayer - 21% CPU (with TFF minimised, 45 when not)