Who doesn't want their site to be faster? I want my sites to be blazing fast, delivering content and design in microseconds... but microseconds is still a dream. Loading in milliseconds is possible with some effort.
Joomla 1.5 loads mootols library by default with every pageload. The mootools.js packages with Joomla 1.5 is located at "/media/system/js/mootools.js" and weighs a significant 72.6 kB. That can significantly lower the speed of your website, especially if the users are on low bandwidth connections.
By the way, if you want to check how fast your site is, you can check it out at our Website Speed Test Tool.
Now that we know that the mootools.js of Joomla 1.5 is big, what can we do? As discussed by Rick, it is quite a complex process to get the right smaller version. However, you can get a compressed version from Google Ajax Tools by going to this link:http://ajax.googleapis.com/ajax/libs/mootools/1.11/mootools-yui-compressed.js - this one is about 64 kB.
While you can reduce the size of mootools required for your site by trial and error as decsribed by Rick, it could be quite painful and may need to be done repeatedly whenever you install a new extension that may depend on a specific component of mootols that you may have removed.
You can get your own mootools.js from the official mootools website or build your own version here.