Who should define software performance targets and how can you define them?

I’m wondering how one should come up with targets of his software performance? How many seconds should a scenario take? How many seconds should take a certain endpoint to reply? Do you set that number before or after doing the development?

Also, who should set that number? Is it the developer that will develop the software or the product that defines the product requirements but not aware of the technical limitations or is it the quality control person?