Do not confuse CI kg/min with FF Kg/min, which is a very common thing.
a 40 Kg/min CI means that for the airline one minute of flight is worth as much as 40 kg of fuel. Therefore, cruise speed will be increased from max range speed until one minute of time saved costs more than 40 kg of extra burnt fuel. When that occurs you are flying at the ECON MACH.
In the 320 this is often difficult to see, since we can only see whole minutes and hundreds of kgs of fuel.