When a browser loads a page, most resources that are referenced in the page are downloaded in parallel. However, this is not true of JavaScript source files. When a <script src="... tag is encountered by the browser, a blocking call is made to retrieve the file. So, if you place your script reference at the top of the page, no rendering will occur until after the JavaScript file has been downloaded. Simply moving your script reference (if possible) to the bottom of the page is an easy way to help improve the apparent performance of your site.
