I've always disagreed with this. In my view you should make it a habit to write optimized code. This isn't agonizing over minor implementation details but keeping in mind the time complexity of whatever you are writing and working towards a optimal solution from the start. You should know what abstractions in your language are expensive and avoid them. You should know roughly the purpose of a database table you create and add the indexes that make sense even if you don't intend to use them right away. You should know that thousands of method lookups in a tight loop will be slow. You should have a feel for "this is a problem someone else probably solved, is there a optimal implementation I can find somewhere?". You should know when you use a value often and cache it to start with. Over time the gap between writing unoptimized and mostly optimized code gets smaller and smaller just like practice improves any skill.