What kind of pc are you rockin'? If you are below the reccomended requirements you'll probably get lag. Depending on how your pc set up you might get lag no matter how much you tweak the in game settings. Like if your processor speed is lower or you have less RAM, it doesn't matter how powerful your graphics and sound cards are you'll still get some lag.
As far as speeding it up you might try running defrag on the hard drive the game is installed on. To run it go to Start > All Programs > Accessories > System Tools > Disk Defragmenter. That should put all your files in the right locations on the hard disk and allow the CPU to access them faster.