When is optimisation premature?

Question

As Knuth said,

We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil.

Answer 1

My question is, if a particular technique is different but not particularly obscure or obfuscated, can that really be considered a premature optimisation?

Um... So you have two techniques ready at hand, identical in cost (same effort to use, read, modify) and one is more efficient. No, using the more efficient one would not, in that case, be premature.

Interrupting your code-writing to look for alternatives to common programming constructs / library routines on the off-chance that there's a more efficient version hanging around somewhere even though for all you know the relative speed of what you're writing will never actually matter... That's premature.