Of course all this needs to be weighed against maintainability and readability of the code. If the code base is not mainly about something very performance crit

Of course all this needs to be weighed against maintainability and readability of the code. If the code base is not mainly about something very performance critical and this kind of thing shows to be a bottleneck, then changing things away from more readable towards performance optimized implementation would require a very good justification. I doubt, that this kind of optimization is justified in most cases. For that reason I find the wording "prematurely breaking out code" to be misleading. In most cases one should probably prioritize readability and maintainability and if breaking code out helps those, then it cannot be premature. It could only be premature from a performance limited perspective, which might have not much to do with the use case/purpose of the code. It is nice, if a performance optimization manages to keep the same degree of readability and maintainability. Those concerns covered, sure we should go ahead and make the performance optimization.