Avoid messages! Messages are the key element of modeling interactions. Nevertheless, writing and reading messages takes its time. Especially if you have a large agent population. Consider, if there is any other way to solve a problem than sending messages.
Avoid redundant memory variables! It turned out that writing the XMLs to the hard drive is one of the bottle-necks in the simulation process. So, the more memory variables you have, the more disk space needs to be filled. Remember that the information written on the hard drive is proportional to the number of agents. Take a look at your agent memory and try to use one variable for as many purposes as possible!