Firewalls are a well-established security mechanism to restrict the traffic exchanged between networks to a certain subset of users and applications. In order to cope with new application types like multimedia applications, new firewall architectures are necessary. The performance of these new architectures is a critical factor because Quality of Service (QoS) demands of multimedia applications have to be satisfied. We show how the performance of firewall architectures for multimedia applications can be determined. A model is presented which can be used to describe the performance of multimedia firewall architectures. This model can be used to dimension firewalls for usage with multimedia applications. In addition, we present the results of a lab experiment, used to evaluate the performance of a distributed firewall architecture and to validate the model.