I created a new game with a realistic population for Earth. 7,287M according to Wikipedia. However, performance is horrible. A single 30 day increment takes between 2 - 5 min. Any thoughts as to the point at which population begins to seriously degrade performance? I know they scale against each other inversely, but it seems there is some population value after which you are basically screwed.
Here is a thread where I tried to document what exactly causes what kind of slowdown in Aurora.Further down there was tested proof that neither population count nor amount of installations on colonies has any say in game speed.(could simply be tested by upping those numbers a couple factors in SM) So those definitely aren't the reason you face the slow-down, and you can probably test it yourself by artificially lowering the numbers.
Tldr version and my guess on the specific problem: Ground units and officers. The calculation burden that the tracking and updating of the numerous officer profiles takes is probably considerable on production intervals, and might be akin to total ship count when determining the slow-down effect of late game empires.(I would recommend to keep it between 15-20 academies total throughout all colonies; - at least the officer window loadup will definitely be pleased with that limit)
I am not sure though if you even spawn with more officers when not starting conventional (, because I
always start conventional), so this might not be your problem right now. That leaves the plethora of ground units as possible culprit, though they only tend to paralyze the colony window loadup, and I have never really been able to sort them out as a general impeding factor.
They still might be one, but as athom, I have also started with 5, realistic 7, and even 10b population, and not seen those problems at all, so ground units seem more unlikely. (maybe I had just "scrapped" them fast enough though)
So my personal bet is that the non-conventional start generated you too many officers, but I am shaky, and there might be something about your game that is entirely different for some reason. Have you created NPR with you for example? Those would spawn with ships, and there you have the reason, because the ship count depends on their population, which depends on your population.
Otherwise, if it is none of those, can you post a screen of your game setup?