american.gxt is the file that includes all the text, like the car names when you get in them, the menu text, etc.
main.scm is the file where the missions are written, the car's coordinates are stored, and it is used for a lot of other things but I don't feel like getting that deep into it
Oh, well processor speed shouldn't have much to do with it, what video card do you have?
If it works in single player I don't know what to tell you though.