Performance requirements are not "the system must run fast". What they will say is things like "the user interface must respond to a request from the user in a bounded time of x seconds". You may think that we are splitting hairs here as essentially we are saying the same thing. Consider photoshop as an example. It will have some performance requirements in it to say that its user interface must respond in a reasonably bounded time. It will not say that everything will run fast purely because a lot of what it does is mathematically very complicated. As such saying that a feature such as resizing a very large image will occur "quickly" has no meaning. What if I'm running it on the minimum requirements? Should it always run at the same speed? Clearly that is unrealistic.

Performance is very important in a system but the requirements set out must be realistic. They must not be idealistic goals. They must be clear, realistic and measurable. If they are not then they will not be met in the implementation phase. If they are not met then the developer will not get paid.