The pervasive presence of portable electronic devices and the massive adoption of the Internet technology are changing the shape of modern computing systems. Applications are being broken down into smaller components that can be modified independently and that can be plugged in and out dynamically. All this drives the development of flexible, high scalable middleware. An interesting category of such middleware supports the eventbased paradigm. In this paper we focus on the scalability issues we have faced in the development of an event-based middleware called JEDI. In particular, we focus on improving performances of such middleware when the number and distribution of components grows. In order to evaluate design alternatives, we have taken a simulative approach that has allowed us to analyze the design alternatives before actually implementing them.