Germanwings and Eurowings and Stat Alliance airlines do not pay the airport for the Priority Lane and thus it is not really taken care of. DUS would handle is differently if the airport flying out of A would pay for it.
It is a bit hit and miss; there are times, when the priority gates are curtained up from the main checkpoints. But I agree, there could be long lines and the scanner at the entrance to the prioirty gates take every boarding pass and sometimes the regular line is even faster than the priority line.