"Lag" is a very broad term. It falls into two major categories. Game lag occurs when the computer is having resource issues. A DXDIAG output would procide enough information to start helping with that, but if the cause of resource issues is the result of some other application running on your computer, a DXDIAG will not show that.
Network lag occurs when any number of routers on the Internet are simply overloaded. This particular version of lag can happen anywhere between your computer and the game server. A PingPlot would help narrow it down.