There is a certain kind of project where you're not sure, at the outset, if what you want to do is even possible due to performance issues. Obviously, that question needs to be answered as early as possible, and so the project is about "optimization" from day one.

A good example is id games -- Quake, Doom, et al. They design games around technology. They need to figure out exactly what the engine can do before anything else.

Phones and tablets are making this sort of project common again as people push the limits of those platforms.