I dont understand why you are saying "you need to repair" this etc. More buildings = takes more cream to process. Of course there might be some performance issues to be resolved but i highly doubt they can make the fps stable inside and outside cities. Theres just so much for the computer to handle in the cities.

Ive got quite a shit computer: Phenom 2 965 BE oc 3,8 ghz, 8gb ram, radeon 6950 oc and still experience the same issues, question is how much its possible to fix.

My tip is to mess around with x.vars.profile file (open in notepad). Ive changed theese.

sceneComplexity=100000;
shadowZDistance=250;
viewDistance=1000;
preferredObjectViewDistance=500;