The increasing complexity of innovative real-time hardware/software systems forced industry to consider system-level design methods. Before actually implementing a system with hardware and software components, system-level design methods enable analysing the performance of different design alternatives that realise the required functionality. In order to develop performance models early in the design process, the parallel object-oriented specification language (POOSL) can be used. POOSL is an expressive modelling language for analysing complex real-time distributed hardware/software systems. Being equipped with a formal semantics, POOSL ensures unambiguous execution of models and proper application of performance analysis techniques. This paper discusses the use of POOSL for analysing the performance of a network processor. A network processor consists of components that perform their behaviour in a synchronously concurrent way, whereas POOSL is based on an asynchronous modelling paradigm. In this paper, we illustrate that constructing abstract models of synchronous systems for the purpose of performance analysis may benefit from an asynchronous modelling approach.