Why are some games unplayable on my Computer?

Could be the game itself, check to see if there is an update for it. I know some games are just laggy, glitchy messes. I had many problems with Gothic 3, which was shared by everyone. Computer could run games that were much more graphic intensive, but had problems with Gothic 3. So I'm guessing its the game. Do a google search for "Divine Divinity lag" or glitch, etc.