I posted this in another thread already, but i´ll repeat it. I did "tests" like
sirphoenix myself, and I ran the game on an old 128mb x550 as well as on a brandnew 512mb gt9600. I tried resolutions between 1600x1200 and 1024x768.
The framerate on both cards is OK on small, and horrible on huge maps. By huge, I mean 100+ Planets, 10 Players, and 10+ hrs played. Changing Resolution, Quality or the imgame zoom level didn´t show noteworthy effects.
Also, the fps kept decreasing the "older" the game got.
Therefor, and by checking RAM/Swapfile/CPU usage, I am rather sure to conclude that
a) Sins eats a truckload of mem (naturally scaling with size of map and therefor # of object)
Sins needs large amounts of processing power to calculate "hidden" stuff, like trade/refinery ships, Hangar Defense Squads, Resource Asteroids etc.
Not being a programmer I can only guess a solution: implement multithreading/processing by shifting AI players or "static" calculations (i.e. income) to other CPU cores, and find a way to decrease the calculation load on trade/refinery ships (maybe by using "fake" phasejumps or something).
Not sure how you could reduce memory usage either, but I *guess* you could save a few bytes here and there by not simulating every little aspect real-time, again this would come to stuff along the lines of income and pop growth .