In this paper, we propose an efficiency/performance measure for dynamic networks, which have been modeled as evolutionary variational inequalities. Such applications include the Internet. The measure, which captures demands, flows, and costs/latencies over time, allows for the identification of the importance of the nodes and links and their rankings. We provide both continuous time and discrete time versions of the efficiency measure. We illustrate the efficiency measure for the time-dependent (demand-varying) Braess paradox and demonstrate how it can be used to assess the most vulnerable nodes and links in terms of the greatest impact of their removal on the efficiency/performance of the dynamic network over time.