A frequent source of confusion I come across is the difference between performance and scalability. They are related, but not the same thing.
Performance is speed – for example, how fast does a web page come up for a user in a web browser.
Scalability is about how much load you can throw at a site before performance degrades to an unacceptable point, or whether you can just add more hardware and keep performance the same.
So yes, performance and scalabililty are frequently related. But no, improving performance does not automatically improve scalability; and improving scalability does not automatically improve performance. It might, but it is not guaranteed.
I think this is an important distinction and you have articulated the difference between the two quite nicely. Thanks for the post.