Yes, in practice, I would say that it’s better to minimise the number of subscriptions, since every one of them needs more infrastructure and checking in the server, especially. Also if you have several items in one subscriptions, their data changes will be packaged into the same data change notification message, which takes less communication bandwidth. Of course, very big subscriptions may lead to too big messages (but that means thousands of items). We have not really measured any performance differences, though. I suppose some balanced design would give the best overall performance.

Anyway, I would suggest that you design the client application according to the most suitable application logic and group items that are logically connected to the same subscription. If you notice any performance issues, then you can start optimising the design from that.

In general, the communication is very fast locally, and the optimisation may be more necessary, if you have a slow network connection between the applications.

Fine. And please, if possible, report back here if you manage to improve your communication with this.