This article proposes a hardware/software partitioning method targeted to performance-constrained systems for datapath applications. Exploiting a platform based design, a Timed Petri Net formalism is proposed to represent the mapping of the application onto the platform, allowing to statically extract performance estimations in early phases of the design process and without the need of expensive simulations. The mapping process is generalized in order to allow an automatic exploration of the solution space, that identifies the best performance/area configurations among several application-architecture combinations. The method is evaluated implementing a typical datapath performance constrained system, i.e. a packet processing application. Categories and Subject Descriptors C.3 [Computer Systems Organization]: Special-purpose and Application based Systems,Real-time and embedded systems General Terms Design, Performance