And, ultimately, I prefer EU3's end-of-year and end-of-month calculation time to a homogenously slow game.
I could live with that too, if perhaps some calculations for POP migration, promotion, factory viability, or assimilation could be done on a monthly basis.
Another thought (though I'm not sure how feasible this idea is, or how beneficial it would be performance wise) would be to maybe somehow stagger these calculations. In other words, instead of trying to crunch all the numbers everyday, only a subset of them would be calculated each day, with the whole lot of numbers being crunched every few days.