I found that retrieving IMAP emails is delayed compared to using POP. In this forum, it mentioned to go to Menu>Accounts>Diagnostics and add in the Advanced Options > Parameters, the following:
–imap-avoid-search-by-messageid
That fixed the delay. However, as we manage a SmarterMail server, is there something that should be configured on the SmarterMail server so that this parameter does not need to be entered for each user?