There's no way to tell if the app pool is shutting down without watching the process on the server in the Task Manager, or viewing the server event logs.
Are ALL pages loading slow, or initial page loads?
Are you on shared hosting? Is your shared hosting have a RAM resource cap? IF your app pool is limited in RAM usage, when YAF loads, if it exceeds the RAM limit set on your pool, the pool will recycle.
Please verify your server hosting limits for the YAF application pool. Something is misconfigured most likely or you are under a common restriction from shared hosting. YAF is a large application that takes large advantage of database and page caching. These take RAM in order to be effecient.
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