Can you help me with this problem?

My son and I both have computers and separate Internet accounts. We both play Forbidden World and other mmo games. I have no problem going through the town or doing the quests but my son's computer lags extremely bad! Is there anyway we can get his pc working better so he can play the game like me. He is not a kid, but 35 years old, and it is very aggravating. We also live at the same place so we can't figure out why his computer is working so badly. Any help or suggestions would be greatly appreciated. Thanks!