It stands to reason that the more EPs in the game the more system hungry the game will get.
There are a few things that you can do to try to help this, basic system maintenance for one thing is a really good idea after a new install. So cleanup your disk, there is a very good and free program just google 'CCleaner'. This has an expanded and simplified form of basic disk cleanup programs, once that is done, there is an option on it to cleanup your registry, that is always a good idea to get rid of registry items that are no longer valid or that may have been corrupted by malware, spyware or viruses. Just ensure that you backup your registry when it prompts.
There is also an unistall program menu on there which runs far quicker than the operating system version. It is always a good idea to get rid of programs that you no longer use.
See what is 'running in the background' certain programs insist on starting up with the computer when they do not need to. An option in CCleaner allows you to disable programs from start-up, be careful with this part, I suggest that you only delete or disable programs from this part if and only if you know what they are.
If you are running Vista then it is already power hungry, the best way to counter that is to google how to speed up Vista, it will bring about lots of going into settings and basically stopping all the bells and whistles, it may not make for a pretty looking computer but it sure will help performance issues.
After all that run another disk cleanup and then run a defrag, again the same company as the CCleaner has a nifty little program called defraggler which is an excellent defragmentation tool.
If your game is still running slow after all then I can only assume that your computer is running at or very close to the basic system requirements and it may need some upgrading for smoother game play.
