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In this paper, an approach to evaluate time performances of networked control systems (NCSs) is presented. Switched Ethernet with a client/server protocol is considered for communication. As a result, the reactivity of these NCSs is not only affected by the network delays but also by the field-device due delays. Obviously, all of them have to be taken into account to evaluate efficiently the NCS reactivity. Thus, by including some preliminary results, we propose an overall study in an effort to achieve this aim. First, we model the traffic of the NCS and provide formal proofs to characterize the scenarios leading to the worst delays. Subsequently, we develop a virtual-queuing-based algorithm (VQA) to search these scenarios and deduce the corresponding delays. Through a practical case study, it is shown how VQA fits experimental observations both quantitatively and qualitatively.