I've noticed this behavior on sites where embedded Youtube videos are being played back with HTML5 video instead of Flash Player (i.e. Duck Duck Go search engine video search results). Also seeing it happen on Netflix.com, although they did mention Netflix would use HTML5 "Premium" video for playback in Yosemite. Performance is horrible right now however, even on my more than capable 2012 MacBook Pro 15" (non-retina).
I tried to confirm if there is a way to disable HTML5 video for playback in Safari but no dice so far.