Sorry realized that of course memmapping the mask isn't an optimal solution here. Numpy does not really have much to help loop through the array in chunks (which would be the cleanest way), though you can of course do that by hand. You might actually have some success with numexpr which always does its calculation in chunks for speeding up numpy, but I did not try this.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.