For numbers near 2**32, factoring a prime number seems to take 150 times longer
I'm not sure that I understand this? Factoring a prime how?
(I should also admit that I don't know what a "n-order wheel" is, or why it would benefit me to use one :)
Also, as I'm limiting my inputs to 0 .. 2**32, I only need the primes upto 65537 in order to factor that range of inputs. Right now, I am trading a little space (130k) to gain speed by building an array of the first 6534 primes at compile time.