I have used the Aeotec Multisensor 6 and if it is plugged in with USB power it sends updates very quickly as status changes. I use this as my main humidity sensor to turn on humidifiers during the winter months when the humidity plumits where I am at.
I also have a Aeotec Aerq sensor and though it isn't nearly as fast to report, it can be used under certain circumstances. It reports at most every 15 min and is only battery powered. I placed MS6 next to it for a period of time and the numbers were pretty much spot on except for the slower changes. I am pondering using this to activate and deactivate the fan in or bathroom. The nicest thing about it is that it is super small.
I also would suggest you look at the homemade sensors from @iharyadi as they are very fast to give updates of status changes. The thread for them is here They also perform the task of a Zigbee repeater and depending on which one you get can even have custom sensors added to them.
