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.

The two metrics are frequently related. It is common that if you improve the performance of your site you can take more capacity – it frequently helps your scalability a bit as well. But it is not guaranteed to be a direct relationship. For example, page speed may be improved by merging CSS or JavaScript files improving performance – but there is no change in load on the backend web server so zero scalability benefit. Scalability is also affected more by resource contention bottlenecks. Improving performance won’t help scalability if your application is blocking on locks from other sessions (unless you are reducing the length of time you hold the lock).

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.