It is quite easy to get confused between scalability and performance architectural concerns because scalability is all about ensuring that no matter the number of users consuming the application, all get the same pre-determined level of performance.
Performance relates to application features that ensure that the application caters to predefined response time and throughput, scalability refers to having provision for more resources on a need basis to accommodate more users without sacrificing performance.
It is better to understand them using an analogy. Speed of a train refers to the performance for railway systems, however, accommodating more trains to run in parallel with the same or higher speed will be referred to as scalability of the railway network.
