Why multiplayer after 200 population is starting to lag?

After reaching about 200 of population, multiplayer is starting to lag little by little. Whenever you reach 400 - 500 lagging is extremely annoying. Is it possible somehow to fix it?

its like the game is CPU limited to a single thread/core making it fundamentally limited by how much you can push through that single thread/core

Consequently it can’t be fixed. This is what you are saying?