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

I don't think that recognized best practices are premature optimizations. It's more about burning time on the what ifs that are potential performance problems depending on the usage scenarios. A good example: If you burn a week trying to optimize reflecting over an object before you have proof that it is a bottleneck you are prematurely optimizing.