There are certain issues through which you can’t upgrade your Joomla 1.5 version to the latest. May be you have integrated some external CMS, random changes in DB structure or any other major changes which doesn’t let you to upgrade your website built in Joomla 1.5.
But on the other side keeping in mind countless user review, it has been proved that Joomla 1.5 is loading slower compare to other CMS even the new version of Joomla 2.3 or 3. Most of the web-developer want to make thier old CMS faster, i was facing the same situation with my 2 websites having Joomla 1.5
The easiest and effective solution is to compress your Mootools.js file to be load faster, the file is located
You can compress your mootools.js with http://dean.edwards.name/packer/, copy the code from your mootools.js and paste here, compress the file update the code in your website.
Say thanks, if it works for you..