OK, replying in case anyone is experiencing similar problems. Low graphic settings seemed to be the issue causing more lagging. The lower settings I was playing with both SP and MP, the worse the lagging was. I updated my graphic card drivers and raised the graphics to medium settings. With updated drivers and low settings, my game was still lagging horribly. Higher graphic settings stabilized my overall performance even though I still experience mild lagging but it is now stable enough to be able to enjoy SP if not completely MP.