I am really not sure about this. Inputs go slightly above 1M, and determining whether a number of magniture 1M is prime is not that expensive, or difficult. Some optimization techniques are to be expected for a 5 kyu kata, but this kata requires only very basic ones, nothing really fancy.

how on earth is this an issue

I've seen hacky code before - this isn't hacky in the least. If anything, it's the most idiomatic way to do this, imo.