The law in Massachusetts limits you to 2 pounds of black powder -- or rather, 1.99 pounds -- same with cartridges and primers -- 999 primers, 9999 catridges, etc.With a permit from the fire dept, a permit no fireman I spoke to had ever heard of, you can store 3.99 pounds.
So necessarily, when I lived there I did all my priming with 2F. While I have no basis for comparision since I've never primed with 4F, I can say that I never noticed a delay worth noting on my .54 using 2F to prime. Compared to a percussion gun, it was slower of course but not much. This is by no means scientific, you could certainly experiment with screening out finer powder or something, but I was satisfied with the speed 2F priming gives.
I thought Powder Valley was in Kansas, I guess they must not have a store front? Kansas is a beautiful state by the way. My first night visiting I experienced the most picture perfect prairie storm, purple lighting filling the sky, pounding rain, and the wail of the tornado sirens at 1am...An EF2 that the locals slept through.