NIS Slows Down E-mail retrieval on Outlook 2010?

When I first start Outlook 2010, it takes anywhere from 3 minutes to 10 minutes for Outlook to start receiving the first emails from the server.  During this period there is considerable disk activity as indicated by the disk drive LED and by the sound of the drive.  On Outlook the send receive status in the task bar area shows something like "Receiving message 1 of xx (33B of xxxKB)".  The byte counter increments by 33 bytes about every 10 seconds.  What is NIS doing when it first starts up but before any emails are retrieved??  When I use my Android cellphone or my iPad (both on the same wi-fi network as Outlook), the emails are immediately retrieved.  Recently I read about this on the web and the suggestion to try: disable NIS e-mail scanning.  I did that and now Outlook starts immediately and starts retrieving emails immediately!  How nice is this!  However, my email is no longer being scanned for threats.

I have compacted my .PST file (Outlook) and archived my emails.  There are no huge email folders.  I probably have about 30 folders.  Nothing exceptional that I can think of that might be causing NIS to slow down.

I am running NIS 2014 on Windows XP SP3 with Outlook 2010.  I am using two POP3 email accounts.

Questions: 1) what is NIS doing that takes so long?  2) is there a solution that will allow email scanning but not slow down Outlook?

Thanks!

Mike