Hopefully someone can help!

I have upgraded all users in a company from Office 2007/2010 to Office 2013, along with a new Exchange (On-Premises 2013) server and a NAS box. All users have no issues except when handling attachments in Outlook.

Attaching a file causes a 15-17 second delay to bring up the Explorer/Browse window. Occasionally, Outlook will "go white" or "opaque" before bringing up the window.
Opening an attachment causes a 15-17 second delay before the relevant program (Word/Excel etc.) is launched. For some users (but not all) this only happens once - subsequent attachment opening is nearly instant.

I have tried (but not in this order):

Emptying the Outlook temp files.
Emptying the PC's temp files.
Reinstall Office.
Disable all Add-Ins.
Run Outlook in Safe Mode.
Run Outlook as Administrator.
Disabled all AV (ESET).
Run KB2989671.
Switched between Cached/Non-Cached Mode.
Reset Nav Pane.

I also tried installing Office 2010 on a test PC - Outlook (2010) worked flawlessly!
Browsing using Win Explorer is like lightning, with no lag whatsoever.

Has anyone got any

It  must be an add-in issue,try disabling addins one by one and check for it.

Try opening the application in safe mode and check if you are able to attach the files,

Click start->all programs->accessories->Run->type Outlook /safe

If it works fine in safe mode, try disabling the add ins for the application and check the result,

1.       Click the File tab. The Microsoft Office Backstage view opens.

2.       Click Options; the Options dialog box appears.

3.       Click Trust Center, then click Trust Center Settings.

4.       In the Trust Center, click Add-ins->Com addins.

5.       Select or clear the options that you want

Hi Tusharverma,

Thanks for the reply. Unfortunately, it does the same in safe mode.

I have also tried disabling all Add-Ins and A\V and there is no difference - I still get a delay. What really confuses me is that it works perfectly in Outlook 2010!

For those having the same issue, I have managed to solve it.

I traced it back to an out-of-date GPO which was redirecting users Favorites. After modifying the GPO, opening attachments in Outlook suddenly sped up!

I'm guessing that the program Outlook calls to open the attachment (Word/Excel etc.) looks at the Favorites as it opens.

Hope this helps someone else.