Premature optimization may or not be the root of all evil, but optimizing the wrong thing certainly doesn't help. When we talk about optimization we're usually talking about minimizing time, and the example says that if you have two sub-processes and one takes 9.9 seconds and the other takes .1 seconds then you should spend your time on the first one, because even if you manage to eliminate the second no one will notice. And that's an important thing to remember.
But optimization goes way beyond that. What if that process was part of a system that was only used by 1% of your user base, or the system had been retired? What if the question isn't what function should I make faster, but which bug should I fix or feature should I implement next? It goes back to one of my favorite questions. "What are you really trying to do?"
Because in almost all cases, what you are really trying to optimize out where to spend your time to provide maximum user value and when. Before you can do that you have to define those 4 terms.
And that's where scope comes in. You could be yourself, your team, your organization, ATG, or Uber Technologies. User has the same scope or larger. Are you working on something for internal consumption only, or does it accrue to an externally facing feature? Or is it something even larger like the public safety or the environment?
What does value mean? There's dollar value, but that might not be the right way to measure it. It could be time, or ease of use (customer delight), or more intangible, like brand recognition. And when will that value be realized? Will it be immediate, or will it be much further down the road?
Those are a lot of questions that you need to answer before you can optimize, and sometimes we don't have all the answers. The answers also impact each other. We have a finite amount of resources and you can't do everything.
So what do we do? We optimize what we can see and understand with what we can do and make the optimal choices based on what we know. Then we observe the situation again and make some more optimizations. Then we do it again. And again. Kind of like a robot navigating down the road in an uncertain environment :)