When QuickBooks installs, it sets defaults amounts of memory cache the database manager will use. Sometimes the settings get changed to less than the optimal amount of cache. This can cause QuickBooks to run slowly or very slowly. It is also possible that these incorrect values get “stuck” and seem to keep coming back. What we have observed is that the cache may revert back to lower (incorrect and non-default) values if the server reboots, a Windows update is installed, a QuickBooks update is installed, QuickBooks runs a utility on the company file and possibly other actions.
For more information visit the Intuit support article regarding this http://support.quickbooks.intuit.com/support/articles/SLN65343