Some AJAX sites fetch all the data and then display it in an interactive fashion. Others will fetch a reduced set of data and then pull the missing data when it's requested. It's the latter where care needs to be taken.
In analyzing how AJAX works, Billy observed that as good as lazy fetching was from a users point of view, this must implemented carefully. Too many fetches asking for very small datasets can over burden an application server. When that happens the server will not be as responsive. This effect will negate the advantages of using AJAX.
Each call to the server has fixed overhead. Each fetch from the database has a fixed cost. RPCs that request small amounts of data generate the highest overhead in these terms. If they were combined to fetch larger blocks of data with fewer requests then the overhead is lower.
The net result is that developers have to be mindful of these two stressors as they integrate AJAX into their web enabled applications. As a final thought, Billy acknowledges the advancement offered by AJAX but opines that it is time to start talking about patterns for developers to follow. Since patterns come out of an aggregation of similar use cases, we can start the process of identifying potential patterns by asking; are you using AJAX today and if so how is it being used?