By default the Portal Super User and Reader User accounts use the NT Authority\Local Service and all SharePoint service accounts should be isolated.
When a file is checked out, a query is made to this checked out version rather than the published version. The issue is that this creates a query to the checked out version and then the system cache makes another query to the published version of the file. As result this creates unnecessary requests which lead to performance issues. Having a reader account helps isolate this behavior.This may also display as a Warning in the Application Windows logs Web Content Management Source.
Add a Domain Reader and User account to the Farm