There is a very useful way to optimize Magento site speed is deferring JavaScript loading till the last moment after all HTML and CSS. It means that JavaScript files are put to the end of page, therefore the web browser will load HTM and CSS before loading these JavaScript files. This leads to loading time be reduced and magento site perform faster than before.
To make this JavaScript parsing easier, we had better not do it manually, let's use an extension instead because of convenience and simplicity.
For example:
https://www.magentocommerce.com/magento-connect/defer-javascript.html