Web Application Optimization

We already have an application built. Using PHP/MySQL on RedHat linux box having AJAX. Most of the programming is completed. The application works fine as well, except for the fact that the AJAX calls are extremely slow. And we need to optimize this part of the application. ## Deliverables I have implemented caching in the back-end and the front end. Example: Alternative PHP Caching, Cache_Lite, and Squid. But I am not really an expert when it comes to Ajax or JQuery. I need someone to help with optimizing AJAX calls and just AJAX in general. The amount of AJAX being called isn't that much literally 15 ajax calls. But, I need to know if there is a better way to make the calls or if any other optimization need to take place other then AJAX. Also, their is some other programming required, such as implementing a "processing window" while the processing of ajax calls and anything else on the page is taking place. May be even by using [login to view URL] plugin etc. Also, I'd also like it if you can advice/implement best way on Securing the web application. There are only 2 main areas where the majority of the processing is being done. Creating and Editing, these are 2 pages, where bulk of AJAX calls are utilized and majority of the processing takes place. Rest of application are reports etc.