@rossko57 has a nice write up on autoupdate here.
I will guess that you see a delay because the device needs the time to decide if you are going to press it again for a double click or hold it for a long click. But it could be something else.
Do you have any other device or sensor that you can also press at the same time to see what it does? Any Z-Wave switches? Anything that will cause a report in the log. I suppose you could be at the console (are you running on a Linux box?), type in ‘date’, and press your button and the enter key at the same time, or as close as you can get. Record the date/time on the screen and compare with what you see in the log file.