Tech Guru Said..
I am explaining this in the context of Web Application Architecture. Scalability and performance are distinctly different. When we talk about improving a site’s performance, what I mean is decreasing the time it takes for a particular page to load or for a particular user-visible action to complete. What a single user sees while sitting at their computer is “performance.”
Scalability, on the other hand, has to do with how many users a site can support. A scalable site is one that can easily support additional users by adding more hardware and network bandwidth (no significant software changes), with little or no difference in overall performance. A site can be slow and still support thousands of users.
If adding more users causes the site to slow down significantly and adding more hardware or bandwidth won’t solve the problem, then the site has reached its scalability threshold. One of the goals in designing for scalability is to increase that threshold; it will never go away.