Again, not saying it’s irrelevant and it might be highly relevant in some problem domains. But in general big-O of the common data structures should be memorized with the rest being trivia that should be accessible in the documentation for the data structure (or by looking at the implementation and doing it yourself).

I take it that you didn't bother to read the thread you are replying to? There is no reason why you need to have the time complexity of any given function memorized should someone ask. You can – better yet, they can – read the documentation or, failing that, read the code to answer the question. If you happen to have it on hand, that's great and all, but it is only necessary if the asking party see themselves as a trivia master.

> Devs not understanding how to optimize to avoid slow time complexity is definitely one of the first nuts you have to crack in a new engineer.

You never really crack that nut, no matter how experienced, as many problems have no known faster time complexity solution to provide the 'how'. To begin to try to crack that nut you're going to down the road of researcher, not writing day-to-day software, and even then your research is not guaranteed to yield results.