Final answer:
The requirement for a system to process transactions in under 3 seconds is a performance requirement, which is a type of non-functional requirement focusing on the system's operation efficiency and user experience.
Step-by-step explanation:
The sentence "The system should process transactions quickly, in under 3 seconds" is an example of a performance requirement. Performance requirements are a subset of non-functional requirements which specify criteria that can be used to judge the operation of a system, rather than specific behaviors. They focus on aspects such as response time, throughput, availability, scalability, and efficiency. Therefore, the correct answer is 4) Performance. This requirement is client-oriented and prioritizes the user experience by ensuring that the system's operations are executed swiftly enough to meet user expectations and requirements.