An update. VDO Germany came back to me in 3 days, and offered to send me a new docking station. Good news and service to me.
This is what their representitive had to say about my findings of the delayed speed, cadence and Watt display:
Regarding speed display and delay:
The z is using digital wireless transmission. That means a complete Data package with the digital coding data plus the speed information is send every few seconds.
The receiver is receiving this data package and needs some time to show first speed.
When you stop, the receiver at least has to wait for the next package plus a certain waiting time, before the receiver finally can detect: No more package coming in. Speed is zero.
We have a similar situation with WATT.
WATT is updated only every 10 seconds, because the WATT calculation needs speed, distance, gradient, change in altitude.
These data is measured by the main unit in different time slots. So for the WATT calculation a complete set of data has to be collected first, before the first WATT-calculation can be brought into display. WATT calculation is using a gliding average algorithm.
So when you start it is collecting data and builds up the WATT-display.
When you slow down, it takes a while before the WATT-display shows a reaction.