Performance measures how fast a system responds to requests under current load:

• Response time for individual requests
• Throughput (requests per second)
• Resource utilization (CPU, memory)
  Scalability measures how well performance is maintained as load increases:
• Vertical scaling: Adding more power to existing machines
• Horizontal scaling: Adding more machines to handle load
  Example:
  A system might have good performance (responds in 100ms) but poor scalability (response time increases to 5 seconds when user load doubles).