Stakeholder says: 'The system must be fast'. Is this a functional requirement? How to document it in the technical specifications?

The requirement "The system must be fast" is a non-functional requirement because it describes the quality of the system, not its functionality.

To document this in the technical specifications, it is necessary to specify what exactly is meant by "fast":

• Indicate performance metrics (e.g., response time no more than 200 ms)
• Define measurement conditions (load, type of operations)
• Establish acceptance criteria

Example formulation:

The system's response time to a user request should not exceed 200 milliseconds under a load of up to 1000 concurrent users.