People i got the solution. At least for me. I have Dell laptop with i7 processor and GeForce GT 630M graphic card. I faced a problem of low FPS even at all lower setting. As a solution i updated GeForce GT 630M driver and problem solved. Now i can play at all high setting with minor lag.
Verdict: If you have good possessor and graphic card which are capable of playing AC4BF and facing lag issue, just update driver and you are done.