This is probably one of CR's oldest tags, next to refactoring and best-practice, both of which have already been removed. I recall similar discussion mentioning that specific optimization tags would be more beneficial, such as for speed and memory. Even if we were to create some of these, we may still end up retagging a lot of questions. On the other hand, maybe it just wouldn't be worth it, as it's just another meta tag, which we're trying to keep to a minimum.
It has a tag wiki which explains its use, but it reveals that it's quite general. It has over 1000 questions, and I'm certain that many more questions would fit well with it. This could suggest that while it may not be applicable to every question, it's still likely being used when the author mentions optimization.
Knowing that every question implies an overall review of the code for improvements, does optimization really add anything even slightly specific? Other tags such as algorithm and performance may not seem too different, but they're also not that general. In other words, it's not as likely that they will just be "thrown in there" when just a general review is requested, which should be minimized.