Loading script asynchronously
One thing you need to know is that your script tag block the rendering of your page. In fact, it blocks literally anything from happening, when you are downloading and executing one script, not one css file or one image is downloaded. So imagine you got 10 script files loaded in your head with around 300k in size, well the browser need to load them one by one (well in fact 6 at a time in FF and WebKit, and as many as 18 (or more) in IE8. In older browsers it will be one at a time. ), and execute them one by one, by that time there is a good chance your page would have already render the HTML and CSS.
The first and very easy thing you can do is loading them just before the body end.
Going one step further with labJS