I was quoting a post from a previous thread on another forum about the PC game Age of Empires II slowing down after long games.
Think about it. If your playing say 4 v 4 michi at 200 pop then our computer has to animate 1600 units, update all those units stats as well as building stats. I know that example is extreme. But I think the a game has a live span. It depends on the number of units, map size, the amount of players and populations. Some have more effect than others, perticuly population. As the game progresses all these dictate how much processor time and memory the game uses. The higher/larger amounts of varibles, e.g. populations the faster the game hits a 'peak' in performance, when the game is running with the maximum it can handle. Once other variables increase the game goes passed this 'peak' and performance drops, the game slows. You can't stop this effect, although you can change its 'peak', by lowering settings e.g. as slopyjoe_mess said change your resulation to 800x600, or turn off music, sound, other programs. But for large games this won't make much different as the game will always hit the 'peak' and slow. You could always buy a faster computer, but thats a bit unreasonable just for a game.