When they turn off are they ramping down or is it instant? Could be a ramp rate issue. Best way to find out where the hold up is, would be to turn on the max logging settings for the devices and the app being used. Then hopefully in the logs you can see where the 2s is being lost. If you post some logs (screenshots) people can help figure it out as well.
Also check out my driver for that switch: [DRIVER] Zooz ZEN Switches Advanced (and Dimmers)
EDIT: One other thing as well, if you are not using the button events for anything, if you turn off the scene control setting the switch is more responsive because it does not have to wait some milliseconds to see if you are double tapping, etc, it just sends the on/off command straight away as soon as you touch it.