Issue application.Quit Takes much time to close android application

I have an issue with application.Quit, i close my application when the user clicks on back button
but what happens is that the android application freezes for about 4-5 seconds the truns to black screen , then shows the status bar with the application name then after another 5-7 secs it closes
so the game quitting takes about 15 secs ,too much time.

I dont know the reason behind this , i have google game service and some other assets integrated in my game
should i need to carry out any procedure before closing the game?
is there anything more powerful than application.Quit to kill the game instantly?
Did you face similar behavior before, i tried more than one solution but nothing helped
thank you very much

In my case, the same problem happened after I integrated Admob plugin into Unity. If this is also your case, you can solve by going to the Plugin/Android/AndroidManifest.xml file and delete the entire UnityPlayerProxyActivity and then move the intent filter down under to the UnityPlayerActivity.

If you are facing this problem then I have solution for it.
I have similar problem for my game. Normally in game quit process works well but after adding some plugin I was facing same problem for android device built.

For solution, I have set UnityPlayerActivity as my launcher activity. Now game quit process work smoothly as before.

            android:label="@string/app_name" >
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />