Business requirements typically express performance in nontechnical terms that specify response time. For example, a business requirement for web-based access might state the following:
Users expect a reasonable response time upon login, typically no greater than four seconds.
Starting with this business requirement, examine all use cases to determine how to express this requirement at a system level. In some cases, you might want to include user load conditions determined during usage analysis. Express the performance requirement for each use case in terms of response time under specified load conditions or response time plus throughput. You might also specify the allowable number of errors.
Here are two examples of how to specify system requirements for performance:
Response for web page refresh must be no greater than four seconds throughout the day, sampled at 15-minute intervals, with fewer than 3.4 errors per million transactions.
During defined peak periods, the system must allow 25 secure logins per second with response time no greater than 12 seconds for any user and with fewer than 3.4 errors per million transactions.
Performance requirements are closely related to availability requirements (how failover impacts performance) and latent capacity (how much capacity is available to handle unusual peak loads).