The evaluation of context middleware systems is a challenging endeavour. Testbed investigations suffer from an unrealistic environment in terms of number of users, high implementation effort for changes and questionable portability of results. Simulation of middleware systems is complex due to the high abstraction of implementation. This paper contributes towards the understanding of a broker based context provisioning system based on black-box measurements of a testbed which are further utilised to increase the accuracy of a simulation model. Both simulations and measurements help in understanding the complex behaviour of a context provisioning middleware and enable the evaluation of complex distributed systems. The presented investigations identify significant parameters and corresponding models for the response delay of the key components of a context provisioning middleware and discuss their integration into an overall simulation model.