Good point, and also why the topic of performance is so prone to heated discussions. To paraphrase Knuth "97% of the time, don't optimize except if it's an easy 12% improvement". I don't think there exists a good heuristic that works each time or for all languages. The right decision is left (pun intended) to the programmer that has to live with it. For me the ideal decision took into account the performance tradeoffs of each level of abstraction and chose the "appropriate" one. Obviously, the "appropriate one" depends on an uncountable number of variables including future scaling issues, probability of refactoring/irrelevance, cost/reward of spending the time to optimize the function, ...