That advice is only for micro optimizations. It doesn't make sense to spend a week aggressively optimizing one function unless you know that function is a bott

That advice is only for micro optimizations. It doesn't make sense to spend a week aggressively optimizing one function unless you know that function is a bottleneck and the optimization is worth it. But many developers take that to mean "don't pay attention to performance at all". They have bad architecture, bad data structures, sloppy code that copies data all over the place, etc. Then, even if they do go back and micro-optimize that one hot function, the whole app still sucks.