Asp.Net Web App Performance is slow after Idle time

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ASP.NET4

Hi,

We have a web application deployed on IIS 7.5 target framework 4.0
the application perform slow when leave idle for few minutes for first time and then perform as expected this happened each time application is idle.
With the help of fiddler I found its TCP/IP connection which is taking time about 21 secs whilein subsequent calls this time is 0.
The Idle time out is also set high and connection time out is also high in the IIS settings.
server is - Windows 2008 R2.
there is nothing in the event viewer related to the website.
we used form authentication but the time out for that is also set about 10 hours in the config file.

Can anybody point me to the setting with is affecting the response time after the app is idle for some time.
Note - this was working proper when deployed withing the LAN but this problem starts when deployed out of the LAN or in separate domain.