When outlining a use case to develop the initial use case model during the Inception phase of Unified Process (RUP, OpenUP, EssUP, Your UP), add two NFRs (non-functional requirement) statements to each use case:
“Single execution of the basic flow should/must not exceed N minutes/seconds/milliseconds/.”
“Up to N executions of the basic flow should/must occur per hour/month/day.”
For example, an operator should be able to perform the basic flow scenario in 20 seconds, and this scenario may be executed by the operations team as many as 500 times in an hour.
This is hardly the level of specificity required to design a performance test, but should be at least adequate for the Architects and DBAs to make decisions about the candidate architecture's ability to meet those criteria.
One of the challenges we face is that RUP's Test discipline is singular, but User Acceptance, QA, and the Performance test groups are separate and distinct. Generally, in my experience with several companies, the last test group to be engaged is a technical services team who are concerned with how the new app may take resources from existing production processes. We want to conduct performance testing during Elaboration, but we have no way to execute most of the scenarios during Elaboration, so a promising approach is to tune for the most significant performance requirements. The suggestion above is to help quickly, briefly, identify those scenarios.
Please consider adopting this convention when drafting your own use case model. Or ... suggest a better way!