That's the academic answer, not the practical one. It assumes your time is worth nothing, or that optimization is easy. Neither is true.

Ever try to get management buy-in for a total re-write of an app that wasn't written with performance designed in from the start? It's painful.

Performance, like security, needs to be built in from the start. If it isn't, you can pray for the so-called "80%-20% Co-incidence" to save you, or you can re-write it from scratch using tighter algorithms and faster data structures. Total re-writes cost a lot of time and money; partial fixes tend to end up as cheap hacks.

Unless you have no clue as to what you're writing, just do it right the first time, so you don't have to do it over later. Remember, if your code gets too slow, (and yes, I've seen this happen) it may actually become too slow to properly refactor. If a comparison run takes several days to run, small, incremental changes become very expensive.

If the app is fast and tight, making it better is cheaper, because the cost of testing is cheaper; and the cost of refactoring is cheaper. For a one off script, this doesn't matter; but for a large scale project, performance is more critical than stuffy academics realize. In business, time is money.