Is that the cap of the whole system or can multiple users use it with individual caps of 3.5kbps? That's somehow both faster and slower than I expected.
If one node kept transmitting non-stop, it could push data out at 3.5kbps and no one else in that range can transmit at the same time.
However, in EU there is a legal limit of 1% duty cycle on 868MHz band and collision avoidance mechanism, meaning on average you can send a packet (up to 255 bytes) once a minute.
