Q: FAQ: Security Performance (Z-Stack, ZigBee)

Change in performance and capacity when using ZigBee security.

Security has communication overhead during the joining procedure, since instead of directly joining to a Router ZigBee requires communication back to the Coordinator to fetch the encryption key, which is subsequently passed to the joining device.
Aside from joining the other overhead of security comes in the form of packet headers such that the application payload maximum is reduced from 85 bytes (no security), to 55 bytes (with security).