Game starts using full CPU speed, however after a few turns (or 'seasons' in game)  CPU speed will drop significantly and enter some kind of 'power saving' mode. And it will never go up again. As a  result the game runs slowly and everything sucks...

I have an intel i5-2410 CPU and I am pretty sure it is because of turbo boost, adjusting CPU speed when needed, or CPU temperature is high. The strange thing is, why it can't go back to full speed any more? It is unlikely to happen in other games...