Ok, I agree that PQ is slow. It is possible to calculate a running total of a column in a million row table before the sun burns out though.

I am very not an algorithm person, but I got a huge speedup from a "parallel prefix sum" instead of the obvious sequential approach or the even worse N^2.

I translated this to M by rote and trial and error (page 2): https://www.cs.utexas.edu/~plaxton/c/337/05f/slides/Parallel...

Implementing the parallel, recursive solution got me a million rows in about three and a half minutes.

Fill down (which I had to do anyway to compare) was about 10 seconds.

So...probably not the first choice in this scenario but could be worse?