Joomla! uses mootols to do a variety of things. If your website or components don't use mootools, then not loading it is a good idea.
Go to your template, find the following link in your index.php and insert the following code. Essentially this is removing the link which loads mootols from your hosting server, and loads the compressed version directly from Google. That brings the overall size of each web page request by 50Kb. That may not sound like much, but in the overall scheme of things if you are obsessively working on reducing the load time of your website, then this is a very good leap forward.
unset($document->_scripts[$this->baseurl . '/media/system/js/mootools.js']);