The use of wireless technology is continuously gaining interest in industrial automation. With the standardization of the IEEE 802.15.4 protocol, low-power sensor network protocols have been introduced in this field. Recently, we investigated the real-time communication capabilities of this protocol. This study is now extended by incorporating the security mechanisms as provided by the protocol standard. In contrast to other papers, which studied the effectiveness of these security techniques, we are interested in whether the real-time capabilities are affected by encryption and message authentication and to which extend. Based on extensive simulations, we investigated the interdependency of protocol parameters and available security options. The results can be used for optimally selecting such parameters according to the quality of service requirements of the application scenario.