The correct term is "slow down", since lag is what you say when internet games have bad connection and stuff, so you cant even move and stuff, but its not caused by your computer. You cant lag with single player game, because it is not connected to internet.
Edit: In order to see WHAT is causing it, just have a look at your ping. The hinger it is the more you lag. However, when you have crappy computer, it doesnt matter on your ping, you just cant move good even if youre playing on localhost. That is called slow down.