Dave Johnson have written an excellent article at XML.com about Tuning AJAX. In fact it is interesting to talk about AJAX performance, benchmarking techniques, specially that we are talking about real time rich web applications and we need really to have this "real time" feature.

In AJAX applications, performance is undoubtedly important. Even so, as with any engineering discipline there are compromises to be made. End users will have different operating systems and browsers, applications will have various amounts of code or data to download and process, and there will be different business problems that AJAX can help solve. You need to consider the OS/browser statistics of the target user, and design with performance on those systems in mind. Also, both code and data size have a large effect not only on processing time but also on network latency, which has led to important AJAX design patterns such as pre-fetching of data. The bottom line is that in a commercial environment it is always prudent to let your end users and the business case for your AJAX application direct your choices. Whether you are a single person building an open source AJAX combo box or a small company trying to compete with large AJAX vendors, you need to consider who the end users are and how they use the product.

There is also many other tuning considerations such the usability of the application and how the user interact with the application, Marketability and the possibilities to track visits performance ... This is in addition to the technical performance will add certainly a positive value to AJAX applications development.