Why do so many developers believe performance, readability, and maintainability cannot coexist?

Question

While responding to this question, I began to wonder why so many developers believe a good design should not account for performance because doing so would affect readability and/or maintainability.

I believe that a good design also takes performance into consideration at the time it is written, and that a good developer with a good design can write an efficient program without adversely affecting readability or maintainability.

While I acknowledge that there are extreme cases, Why do many developers insist an efficient program/design will result in poor readability and/or poor maintainability, and consequently that performance should not be a design consideration?