Inbound Mail Takes A Long Time To Process.
It can take longer for inbound emails to be read than the poll time or emails can be left inbox for a long time before they are processed.
Remove emails that have been marked as read from the inbox. Each time mail polls it has to read all emails in the inbox to see if they are read or unread. Sometimes these are left in the inbox and if too many are built up this can cause a performance problems in reading the mail, marking them as read and logging calls