Just because you didn't attend that great session doesn't mean you still can't watch the lecture! Every weekend we host recommended web development talks on Nettuts.
In this talk hosted by JSConf's Thomas Fuchs, he reviews some of the tweaks you can make to JavaScript to significantly improve performance (at least in terms of percentages). Along the way, he covers why you shouldn't pre-optimize your code, how each browser's JavaScript engine excels in different areas, and even how simple things like reordering expressions can significantly improve performance.