If your SharePoint STS does not use session cookie (check by run PowerShell command: (get-spsecuritytokenserviceconfig).usesessioncookies ), you can set cookie timeout. Please
read http://msdn.microsoft.com/en-us/library/hh147183.aspx
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Probably using sessions will be helpful for Sharepoint to log out the user after some time of inactivity. It won't make Sharepoint slower when used Session State for a limited functionality.
Below reference is for an idea to get the work around,
If your SharePoint STS does not use session cookie (check by run PowerShell command: (get-spsecuritytokenserviceconfig).usesessioncookies ), you can set cookie timeout. Please
read http://msdn.microsoft.com/en-us/library/hh147183.aspx
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