There is nothing for us to take away in this discussion. So let me be the first to tune down: all I want to say is: don't take that 300ms as given, it sits in this uncomfortable region too short to be an async op and too long to be noticeable (anything between 50ms and 2s fits this bill). Most likely the query is doing something suspicious and would benefit the most to take a closer look at.
I was totally with you until that last sentence, then you lost me again.
Saying a DB query is too long by giving an arbitrary number is like saying a rope is too long. That’s solely dependent on what you’re doing with it. It’s literally impossible to say that X is too long unless you know what it’s used for.