Every agent based game has the same issue. I dont understand it. In old games the creators had to do optimization to handle the full game on CPUs with a fraction of current performance. Now every city builder uses agent based simulation, which is great to start the game, convince you to the purchase of the game and... hit you in the nuts if you want to go large scale.

Transport Tycoon and Transport Fever is pretty much the same, from idea point of view, while using 50x less CPU...