To be honest, it could be dozens of things.....
post your specs.....
games can lag due to all sorts of circumstances:
- P.S.U.
- GPU
- RAM
- Drivers/Software conflicting, either with each other or game
- Hardware overheating-cooling insufficient is one
- Drivers not up-to-date
- Game not patched up-to-date
- Your system meets min. requirements at very least
- Bottlenecks-caused amongst other things by High-end cpu but low-end GPU, or low RAM and high resolution settings on your monitor, ie bad configuration of your hardware
and so on....