[voidstarcpp]

>But if our performance knowledge is outdated, how do we know which cases to separate the code into?

Case separation is less of a commitment than deciding on the specific optimization yourself. You know empirically which cases are used in your application and can separate those to see if there's a benefit without any manual optimization knowledge, like I did in the image scaling example, without having much of a theory in mind as to why it might be faster.