To summarize, do not strive to write fast programs—strive to write good ones; speed will follow. But do think about performance while you’re designing systems, especially while you’re designing APIs, wire-level protocols, and persistent data formats.
"IADA", or "Identifier, API, Data, and Architecture," is the order of difficulty of changing those aspects (i.e., Identifiers are the hardest to change, architecture is the easiest, relatively), and reinforces this recommendation.
That is, APIs are hardest to change, so focus on getting that right; later on, it's easier to change the implementation to make it faster or more efficient.
