Activating Data Secure on a Group Address adds a per-telegram overhead (authentication and encryption) that reduces the usable application payload.

True. Each Data Secure telegram carries additional bytes for the sequence number, the MAC for authentication and the encryption envelope. Those bytes occupy room inside the standard telegram and therefore shorten the application payload that can be transported in a single frame. For long DPTs such as 14-character DPT 16 strings, this overhead can force fragmentation into multiple telegrams, which is an important sizing consideration for high-rate visualisations.

When you secure a Group Address that carries long DPTs, monitor the bus load — fragmentation can multiply the number of telegrams sent for the same payload and stress a busy line.