I think it's becoming clear now that AJAX enabled applications generate a higher load on an application server than a non AJAX application. I guess customers will have to size their boxes appropriately as a result. The problem is related to the fact that an AJAX enabled pages simply send more requests to the server because of their high level of interaction versus a conventional web page.

High level discussion on whether this is true.

/dev/websphere: AJAX and its impact on servers