If I can suggest, anyone having Lag issues in game should always open their task manager while the game is running and check what might be causing it.
How do I open my task manager?
Right [click] on your task bar and select "Task manager"
it will look like this:
if you click on the % numbers at the top it will put the most resource using process at the top.
I would suggest if your CPU and Memory numbers are above 60% you have too many processing running in the back ground (skype, spotify, Itunes, etc, etc) Close them by right clicking on them in the list and select "End Task"
If you GPU number is high (above 60%)...and 3DX is the only game you have open....lower your graphics setting in game.
if your Network number is high (and its something other than 3DX) then something is downloading in the while you play 3DX...you will see it in the list. Close is as above.