In the panel this code is used:

new Ajax.PeriodicalUpdater("xfdisplay-dashboard", refreshUrl, {
        method: 'get', frequency: refreshTime, decay: 2
    });

decay (Number; default is 1): The rate at which the frequency grows when the response received is exactly the same as the previous. The default of 1 means frequency will never grow; override the default if a stale response implies it's worthwhile to poll less often. If decay is set to 2, for instance, frequency will double (2 seconds, 4 seconds, 8 seconds...) each consecutive time the result is the same; when the result is different once again, frequency will revert to its original value.

http://prototypejs.org/doc/latest/ajax/Ajax/PeriodicalUpdater/index.html

We have a refresh rate at 30 seconds.
So first after 30 seconds is it refreshed, but that it takes 1 minute to refresh and then 2 minutes.

The decay should be 1 IMHO