Computer networks have progressed from a simple store-and-forward medium to a complex communication infrastructure. Routers in the network need to implement a variety of functions ranging from simple packet classification for forwarding and firewalling to complex payload modifications for encryption and content adaptation. As these functions increase in number and complexity, more processing time is required, and packets experience a significant processing delay. In most network simulations, this delay has not been addressed because it was considered negligible. However, we show that this network processing delay can reach the magnitude of long-distance propagation delay and thus becomes a significant contributor to the overall packet delay. We evaluate different network applications and develop a model that characterizes packet processing cost with only a few parameters that can easily be derived from our simulations. To validate our simulation and our model, we compare them to actual network measurements. The contributions of this work can be used to increase the accuracy of network simulations and improve network performance estimations.