As software development cycles become shorter, it is more important to evaluate non-functional properties of a design, such as its performance (in the sense of response times, capacity and scalability). To assist users of UML (the Unified Modeling Language), a language extension called Profile for Schedulability, Performance and Time has been adopted by OMG. This paper demonstrates the use of the profile to describe performance aspects of design, and to evaluate and evolve the design to deal with performance issues, based on a performance model in the form of a layered queueing network. The focus is on addressing different kinds of performance concerns, and interpreting the results into modifications to the design and to the planned run-time configuration.