> Consider the cost of every field, of every action.

This runs counter to most modern software performance principles. Thanks to modern hardware optimisations (cache hierarchy, ILP, branch prediction), modern compiler optimisations (aggressive inlining that leads to a much wider view), and increased concurrency, the notion of some action having a cost lost most meaning about 20 years ago, and increasingly since. Because how fast some action is now depends on a much broader context of what else is going on in the program (and the machine), action X can be faster than Y in one program and the same or slower than Y in another.

Because it's nearly impossible to generalise (and so what was true in your previous program may not be true in your current one unless they're nearly identical), the advice is to first profile your program so that you know how fast or slow different parts are in the context of your particular program and then to focus the optimisation efforts on the hot paths in your program. Otherwise, you may end up spending effort where it makes no difference, and this comes at the cost of optimising what matters, overall harming performance.

Taking responsibility means being smart about directing your resources to where they can have the most impact.