On a side note: not caring about performance “until you find performance issues” is a huge problem with modern software imho. That’s the reason apps are so slow, updates take so long, everything uses so much space. I’d wish that performance would always be a point of consideration, not an afterthought once there are problems.
I agree that junior devs should focus on readability, rather than focusing on everything and just getting it all wrong. but as you say, if you have more experience, performance should be a point of consideration.