You may have previously heard about the practice of treating performance as a first-class feature. Traditionally, performance (along with things such as security, availability, and uptime) was only considered a non-functional requirement (NFR) and usually had some arbitrary made-up metrics that needed to be fulfilled. You may have heard the term performant before. This is the quality of performing well and is often captured in requirements without quantification, providing very little value. It is better to avoid this sort of corporate jargon when corresponding with clients or users.
Using the outdated waterfall method of development, these NFRs were inevitably left until the end and dropped from an over budget and late project in order to get the functional requirements completed. This resulted in a substandard product that was unreliable, slow, and often insecure (as reliability and security are also often neglected NFRs). Think about how many times you're frustrated...