This paper presents a QoS research study on ad hoc sensor networks from a delay point of view when WEP (Wired Equivalent Privacy) security is enabled or disabled. A queuing model with one queue and two servers is used to estimate the security delays in the traffic between ad hoc sensor nodes. An algorithm is designed to estimate the service time delay and the security time delay based on measured timestamps. The main result is that the security adds a significant delay that may affect future killer applications over ad hoc sensor networks like secure medical application, robotics applications, Voice over IP (VoIP) and other delay-sensitive multimedia applications.