I did some tests.
I put a byte counter I a loop without delay.
The nano was able to publish every fourth number. So the network is functioning but for not a very high speed requirement.
I did the same with a MQTT client on a Wemos D1 mini module. The difference in speed was dramatic. No gaps between numbers and much faster.
Because of the envisioned use for controlling a model railroad track layout I decided to go for the higher performance of the Wemos with all negative consequences as far s complexity of decoder composition is concerned.
Thanks for the support!