OS Windows 7
Microprocessor 2 GHz Intel Core i7-2630QM
Microprocessor Cache 6 MB L3 cache
Memory 4 GB 1333 MHz DDR3
Video Graphics AMD Radeon HD 6770M (1 GB GDDR5 dedicated)
Display 15.6" High-Definition LED HP BrightView Display (1366 x 768)
I don't think the above configuration is low. But when i play mafia game i feel the lag. If i turn right it takes at least 2 secs to turn. What should i need to do for making it normal?
Are you playing the game on max settings (tell us your settings)? Try lowering your graphics a bit such as setting anti-aliasing or shadowing to lower level. These two components can really suck up processing time and can lag your game.
Also try turning off Windows Aero or the gadget bar and any unused background applications those sometimes can require unnecessary hogging of your computer (usually it shouldn't cause any problems considering your processor should be able to handle them).
With what you provided I can only determine the causes for your lag is your 6770M (laptop graphics) isn't capable of handling graphic demanding games. HD 6770M isn't a strong card, its mid range average (it is very weak compared to what I have HD 6870).
After few search i figured out that, in BIOS, switchable graphics was set dynamic(default). I changes it to fixed. After this change, when ever i plug in the power, AMD Radeon is automatically selected instead of Intel graphics card. So i play games only when im plugged to power, which isn't a bad choice. Now i don't feel even a micro second delay!