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Separate application pools may help.<br />
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what else is on the server? How much memory does the server have?<br />
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Which web site freezes raising complaints?
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Well, I offered info that any future readers could benefit from, even if somehow the asker has abandoned the question. Is that ever justification to avoid closing a question?
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IIS worker process in the task manager shows, sometimes it is taking more memory and more CPU (Screenshot is below). Sometimes it is freezing the server..
What are the steps to find exactly what website (under the app pool) is taking more memory and more CPU?
It is a windows 2012 R2 server.

How to find websites in the IIS taking more memory and CPU?

Windows Server 2012 is the server version of Windows 8 and the successor to Windows Server 2008 R2. Windows Server 2012 is the first version of Windows Server to have no support for Itanium-based computers since Windows NT 4.0. Windows Server 2012, now in its second release (Windows Server 2012 Release 2) includes Foundation, Essentials, Standard and Datacenter, and does not support IA-32 or IA-64 processors.

Windows Server 2012

IIS is Internet Information Services, the web server included with Windows Server operating systems. All current versions are built on a modular architecture; modules can be added or removed individually so that those required for specific functionality are installed. The full installation of IIS includes HTTP, security, content, compression, caching, logging and diagnostics.