Assume that a URL of a Proxy Automatic Configuration (PAC) file is used in Internet Explorer proxy settings. When Microsoft Lync 2013 (Microsoft Skype for Business) uses the PAC file to connect to a suitable proxy server, the PAC file is downloaded on every HTTP-based proxy request. It causes heavy network and server loads.
This issue occurs because the PAC file for HTTP-based proxy resolution isn't cached by WinHTTP when the URL of the PAC file is explicitly specified in Internet Explorer settings. It causes the PAC file to be downloaded for each HTTP request by Lync 2013 (Skype for Business).
To fix this issue, install the October 13, 2015 update (KB3085581) for Lync 2013 (Skype for Business).
Note Lync 2013 was upgraded to Skype for Business in April 2015.