It seems it travels much farther and much faster when using metric. No wonder they chose it over imperial.

/s

We could probably improve on that significantly without losing speed.

return $x < 8

That should yield one additional correct answer, while also confusing anyone who thinks it just returns false.

And if we just hard coded and checked the first 20 or so primes before always returning false, we would probably get noticeable improvement (depending on the total range).