The terms performance and scalability are sometimes used interchangeably, but in actuality they have very distinct differences. The important distinction between the terms is that performance is a measure of a data point, such as the response time of a request, the amount of CPU or memory that a request needs, etc.
Scalability, on the other hand, measures your application’s ability to maintain its performance as load increases. In other words, if you can service a single request in 500ms, can you service 1000 requests at 500ms each or does the response time degrade as your load increases?