Hi Kartik,
Thank you for reaching out to us.
Kindly note that the AdMob SDK does require a certain amount of resources in order to display ads. Thus, processing time should be expected from it, particularly when fetching an ad. With that said, I would suggest loading ads during non-time-critical sections of the game (such as the menu) in order to lessen the visible impact on the game's performance.
Regards,
Ziv Yves Sanchez
Mobile Ads SDK Team