Very nice kata, but I felt the same about the optimisation - I've completed it in Python as well btw. The description should make the optimisation demands clear, otherwise I've spent hours with my fully working (logic wise) code that would not be accepted by the server. Ended up mindlesly copying prime algorithms from different sites just to see if they improve the speed.
Kata solved now, but all the good feeling about getting the logic is lost after all the obscure and undefined loops on making it pass the server 12sec limit. Nothing wrong with optimisation katas, but please, make it clear in the description. Had to rate it a "sad face" purely because of that, very nice kata otherwise.
Maybe author could relax some of his tests, as big numbers do little to test the logic anyway.

To see the input of the hidden tests, print it. Did you report this as a bug?

I understand your frustation. Usually I keep the tests to half the time given by CW. It's not the fault of the kata if CW runs sometimes erratically. Nevertheless 209 guys passed the kata with Python so it is easily doable when CW runs correctly. I have myself lots of timing-out. Report it as a bug: https://github.com/Codewars/codewars.com/issues.