there might be many cases why your game may lag.
1. Your computer doesn't meet the minimum requirements needed to run the game properly. This means that either your CPU,Graphics Card,RAM Memory or all of these combined are too slow to run this game.
2.You should download a rock solid tool called CCleaner. It cleans your system and prevents it from bloating.
3. Have you tried disk defragmentation?
4. Try Game Booster. It's a free app. Launch the game with the program's launcher. It's a very good tool that will help increase game performance.
Ask me if you don't understand something from the above,and contact me if anything of these worked for you or not.
EDIT: Please don't discuss torrents on this forum.