Pre-load & Javascript Async & Defer performance etc?

Pre-load & Javascript Async & Defer performance etc?

I think DJ-extensions DJ-Classifieds etc work with some pre-load?

See this article from Mozilla how it can work to implemented

http://developer.mozilla.org/en-US/docs/Web/HTML/Preloading_content

I would like to use Javascript Async & Defer also that could improve speed like this free great Joomla plg http://www.daycounts.com/shop/free-stuff/javascript-async-defer

what you think is there any improvement and small update possible without this plg ? Thks.