Why include writing the primes to a file instead of, say, standard output? That increases the optimization space drastically and the IO will eclipse all the careful bitwise math

Does having the primes in a file even allow faster is-prime lookup of a number?

No real reason. It's just an arbitrary task I made for myself. I might have to adjust the goal if writing to the file becomes the lion's share of the runtime, but I'll be pretty happy with myself if that's the project's biggest problem.