Identify Performance Acceptance Criteria

It generally makes sense to start identifying, or at least estimating, the desired performance characteristics of the application early in the development life cycle. This can be accomplished most simply by noting the performance characteristics that your users and stakeholders equate with good performance. The notes can be quantified at a later time. Classes of characteristics that frequently correlate to a user’s or stakeholder’s satisfaction typically include:
Response time. For example, the product catalog must be displayed in less than three seconds.
Throughput. For example, the system must support 25 book orders per second.
Resource utilization. For example, processor utilization is not more than 75 percent. Other important resources that need to be considered for setting objectives are memory, disk input/output (I/O), and network I/O.
Source - Microsoft books