I tried using 6k+-1 for all primes and for some reason it performed slower. I think I have something completely inefficient somewhere, can't figure out where though. I think it has something to do with me increasing k and then multiplying with k while I could have simply added 6 to K...
and I haven't tried with optimization on, first I want to get the algorithm as good as possible before trying to squeeze out those extra milliseconds :)
