speeding up + ctfe question

[maarten van damme]

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 :)