Decrease in performance of my dell inspiron 15r 5521

my dell 15r 5521 recently is experiencing lag during playing high graphics games such as f1 2012, skyrim etc which used to work flawlessly before. When i took the laptop to the service center they replaced the entire motherboard, which included all the components like video card, heat sink etc. 

However after the servicing the lag has only 50% gone and a considerable amount of lag persists which makes the gameplay experience rather painful. Can anyone suggest a possible fix for this problem.