The reason for a valve's extremely slow reaction time in the first week is that the electronic valve has 1,000 steps in from the fully closed position to fully open. This makes it extremely accurate and quiet when controlling to a set point temperature in normal operation.
When a valve is first mounted on a radiator it has to learn the 'on' position, so it can work efficiently and react faster in the future. When it is in this 'learning mode', each time the valve is called to heat a room it start to control to the temperature opening 5 steps, it waits to see if the temperature of the hot water flowing through the valve body (the metal part that the water flows through) changes, and then after another 60 seconds it opens another 5 steps etc.
When mounted on the flow side of the radiator it will normally have established the 'on' position of the valve after about 30-45 minutes, it will store this in its memory and will then fully open as usual to allow the radiator to warm up.
Next time it opens it remembers the position that it need to go to and immediately starts to control around that point.
In the case of it being on the out flow of the radiator, it may reach the 'on' position, but because the water is flowing into the other end of the radiator it will not sense a temperature change, because of this it will continue on the 5 steps routine for much longer. After the first week the valve will have eventually worked out the correct 'on' position but it will take much longer for it to do this in this case.
You can stop a valve from learning if you simply press any button on the valve whilst it is trying to warm up the room. In this case the valve abandons the learning for that time and will wait until the next time it is called to heat a room before it tries to learn again.
Don't forget that if you ever put a valve into 'mount mode' again then it does not know if you have taken it off and put it onto another radiator so it will start it's weeks learning again.