UML Specifications provides some guides for the description of performance requirements, but these requirements are represented with natural language constraints. This paper introduces UML extensions for the representation of temporal requirements and resource usage and their automatic evaluation. They are defined using standard UML extension techniques. These standard extensions introduce a set of formal constraints, tagged values and stereotypes, which allow the representation of general latency and capacity quality of service requirements. We have included these extensions in a commercial UML CASE tool that provides scheduling analysis services and results. We use scheduling analysis and simulation techniques in the evaluation.