In this HA context, “instant status” is usually used to mean a specific technical method by which a mechanical dimmer switch on the wall reports immediately to the hub when that switch is changed. It should be around 250 ms.
Lutron holds patents on this, and any switch, regardless of the specific frequency/protocol that it is using, which wants to have the switch report immediately to the hub has to license the Lutron patents. Control 4 does, Leviton does for their high-end switches, Cooper does. GE/Jasco does not. For Z wave switches, this is implemented with the "hail " Commandset.
(And before anyone asks about a Lutron patent which expired this year, yes, one did, but there is a second one also in force which will not expire until next year.)
That said, Lutron did not trademark the term “instant status” so there are some other manufacturers who will use that term for one of two other different methods.
The first alternate method is Z wave “association”. With this method, the wall switch sends an “on” command to devices in its Association group, which can include the hub. But instead of the hub turning on, it ignores the command and just takes note of the fact that it received it. This does not violate the patent. It’s used by quite a few switches, and you should generally see a process transmission complete around one second, maybe 750 ms. So it’s a little slower than the Lutron method, but not by much.
The second alternate method is available to the newest generation of Z wave switches and uses the “central scene” Commandset. With this, the wall switch remaps the on command to a scene code and sends that same code to the hub for processing. Again, for technical reasons this does not violate the Lutron patent. Homeseer uses this method, and describes it as “instant status” even though it isn’t the same as the Lutron.
It’s not exactly clear just how fast this is. Some reports have said that the Homeseer switches are running noticeably slower than a Cooper switch would, maybe a second and a half. Other people have said that they haven’t noticed any difference, it’s still sub second.
It’s probably obvious by now that if you have an efficient Z wave network, all three methods are going to be fast enough that you probably won’t notice the difference. To be honest, instant status was a much bigger deal back in the third generation of Z wave, but now we’re in the fifth. Everything’s pretty fast.
If a zwave switch is noticeably slow to update status with the hub these days it’s almost always a problem with the network, not the switch.
Even the slowest not an instant status zwave switch shouldn’t take more than a second and a half to update status at the hub.
Zigbee is a little different. The same patent applies but there aren’t the same alternatives. SmartThings gets around this by checking the “heartbeat” of the hue bridge every five seconds. So even there it’s a pretty short time before hub status gets updated.
But with Z wave, most people can’t see much difference.