Charles Eames said that "design depends largely on constraints".

If you know in advance, then one of the constraints on the design of that function is that it's on the critical path and that it has to meet the specification.

You're stating the obvious that you need to consider its implementation.

But that's not the same as "I have a function that is not on the critical path and the performance constraint is not the most important, but I'll still spend time on optimizing it instead of making it clear and easy to understand."