outlook 2013 has a delay when clicking between message

We use Outlook 2013 with Office 365.  A couple of our users are experiencing slow to very slow speed when switching between email in the inbox.  I opened one of the outlook on another computer and also in in Office 365.  The speed was fine.
I opened task manager and the CPU was at 0 and memory was at 39% on the users computer.
Outlook is set for cached mode.

So I'm puzzled what on the computer is slowing it down.  Are there settings in outlook that could be the problem?
J.R. SitmanIT DirectorAsked:
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Vasil Michev (MVP)Commented:
First, try Outlook in safe mode (either hold the CTRL key when launching it or type "Outlook /safe" in the run box). Completely disable any AV software, as scanning modules can cause this as well. Check the size of the OST file, anything above 5GB might cause  problems. Lots of other causes, check here:  https://support.microsoft.com/en-us/kb/2652320
Shweta GoelCommented:

To fix this issue, please try: -

•      Run Outlook in Online Mode.
•      Run Outlook in Safe Mode (Press Win + R > Outlook /safe > Enter).
•      If safe mode shows no issue then you can also try to disable doubtful add-ins to verify which add-ins created this issue.

J.R. SitmanIT DirectorAuthor Commented:
Thanks.  I'll have her test today
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J.R. SitmanIT DirectorAuthor Commented:
one user has a PST attached to her mailbox that when outlook opens it prompts for a password.  Does outlook update the PST when it is running?
Qlemo"Batchelor", Developer and EE Topic AdvisorCommented:
Outlook is not using that PST until the password has been entered. Other accounts are updated independent from that.
J.R. SitmanIT DirectorAuthor Commented:
so when she enters the password does it update the PST regularly when outlook is running?
Qlemo"Batchelor", Developer and EE Topic AdvisorCommented:
A flag is set in the PST (in case Outlook crashes), and any change in Outlook for the folders related to the PST are reflected in the PST file, of course. If the PST is no target of email receive or send operations, nothing should change.
J.R. SitmanIT DirectorAuthor Commented:
J.R. SitmanIT DirectorAuthor Commented:
when outlook opens does it scan the entire inbox?  So an inbox with 500 will open quicker than one with 1500?
Qlemo"Batchelor", Developer and EE Topic AdvisorCommented:
It is not that simple. A bigger PST or OST file needs longer to get checked for metadata when opening Outlook, but Outlook does not scan the inbox. Depending on the view settings, like sorting and filtering, it might last more or less long. But you can't say 500 MB need 10 times more time than 50 MB.
J.R. SitmanIT DirectorAuthor Commented:
ok.  Just looking for clues on our issue.  

safe mode did help.  However, problem cause still under research.
J.R. SitmanIT DirectorAuthor Commented:
turns out the problem was the PST.  We opened a case with Microsoft and the user had a PST connected to her outlook.  It had 21 gigs of data.  So when outlook would sync to it, then it would sync the 21 gigs to Office 365.  Hence using all our bandwidth.

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J.R. SitmanIT DirectorAuthor Commented:
just curious if anyone read my last post?
Qlemo"Batchelor", Developer and EE Topic AdvisorCommented:
So one user caused the issue for other users, as the sync was eating up the usable bandwidth?
J.R. SitmanIT DirectorAuthor Commented:
yep the one users PST attached to her data, was the issue
J.R. SitmanIT DirectorAuthor Commented:
Had to contact Microsoft to solve the problem.
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