To know the loading of an application, you could implement org.zkoss.zk.ui.util.Monitor to count the number of desktops, sessions and requests.

Once implemented, you could register it by specifying the following in WEB-INF/zk.xml (assume the class is called foo.MyStatistic):

<zk>
    <listener>
        <listener-class>foo.MyStatistic</listener-class>
    </listener>
</zk>