I don't see where this is premature optimization. You, and the author, clearly know a lot about the field and the nature of the problem, so you're already past the 'premature' portion. If I know from the start that I've got a function that will be called 10,000 times per second, there's no reason I should write the un-optimized version first. That's like starting every sort off with a bubble sort; I don't need to optimize to quick sort switching off to insertion as appropriate from the beginning of coding, but I know I can do better than bubble from the get-go.

Defining "premature optimization" as the less hyperbolic "optimization before we know we need it," I think it's clear that the problem you've written shows you already know you need it!