Premature optimization includes both what you describe and what the GP talks about. Adopting the general rule, "No optimization without quantification" helps in

Premature optimization includes both what you describe and what the GP talks about. Adopting the general rule, "No optimization without quantification" helps in all cases: validate those assumptions early in the design process (with prototypes, user observations, whatever) and with regard to optimization of code later on, profile it intelligently to identify bottlenecks, don't just assume the issue arise from what seem like the obvious culprits.