frequency means, how often a vehicle comes by a station.
For two diffrent stations it is pretty simple to say that, 1/2 of the frequency is travel time for thatvehicle.
the magic 20 minutes limit does not refere to the travel time for a vehicle, but the time a passenger/cargo is willing to travel. meaning, the walk from his destination to the station and then travels by vehicle and then again walking from the station to the endstation. walktime point A to station+waiting time for vehicle+travel time with vehicle+ walktime station to point B.
With faster vehilce and more stations we can influence the travel distance. with lowering the frequency on one lane we reduce the waiting time. 3 variable which can influence the travel distance.
another thing that i do not yet know, how is frequency calculated with more stations. i don’t think it referes to a whole cylce of a vehicle, meaning how much time goes by until vehicle A reaches station A again.
your example of the cargo is interesting, because that can mean two diffrend things.
1. cargo does not care about waiting time (like you said)
2. cargo production is not actualy frequend but more spontaneously (meaing: it will be produced, when in the next 20 miutes or less, cargo can reach a destination where it is needed, it will be produced)