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

Here's the problem I see with the whole concept of avoiding premature optimization.

There's a disconnect between saying it and doing it.

I've done lots of performance tuning, squeezing large factors out of otherwise well-designed code, seemingly done without premature optimization. Here's an example.

In almost every case, the reason for the suboptimal performance is what I call galloping generality, which is the use of abstract multi-layer classes and thorough object-oriented design, where simple concepts would be less elegant but entirely sufficient.

And in the teaching material where these abstract design concepts are taught, such as notification-driven architecture, and information-hiding where simply setting a boolean property of an object can have an unbounded ripple effect of activities, what is the reason given? Efficiency.

So, was that premature optimization or not?