A key enabling technology for the SOA-based approach is middleware, which comprises of reusable building blocks codifying design patterns. In the SOA-based approach, a system is typically implemented using a composition of a group of such patterns, referred to as a vertical variation. The patterns used in a composition and their configuration options can have a a profound impact on system performance. In this paper we present a model-based performance analysis methodology for a system built using a composition of the Reactor, Active Object and Monitor Object patterns. We implement the performance model using CSIM and illustrate the methodology using examples. By enabling design-time performance analysis, our methodology alleviates many of the disadvantages of post-implementation performance analysis approaches. The methodology can thus provide key guidance towards meeting the performance objectives of a system in a cost-effective manner.