Outlook Slow in Online Mode

Ran into an odd issue with Outlook running slow when in online mode and accessing a mailbox on Exchange Server 2013. Not actually my client, but trying to help out a colleague. Also, not actually my fix, my colleague Patrick figured it out.

The scenario:

• Outlook 2007 clients with minimum updates to work with Exchange 2013
• Exchange 2013 CU4 (SP1)
• Legacy software that makes upgrading client and server problematic
• Exchange 2013 running on physical server works fine.
• Migrated mailboxes to DAG and CAS running as VMWare VMs and Outlook is slow.

As there was a lot of legacy stuff in this environment, every good computer geek likes to blame old stuff. However, that didn't change the fact it all worked fine on the physical server, but not the virtual ones. So, the overall configuration while not optimal, should have been functional.

We looked at standard stuff like misconfigured URLs, certificate configuration, antivirus configuration, resources allocated to the VMs. None of it seemed to be an issue. We changed a few things, but what we did generated no improvement.

Finally, my colleague found some information about Exchange 2013 and recent versions of VMWare Tools causing issues with network connectivity. I've had issues in the past where we needed to update to the latest tools, but nothing like this.

The solution (for now) ended up being removal of the VMWare Tools from the VM. Based on research, this appears to be related to TCPAckFrequency. I don't normally implement on VMWare, but I'll be watching for this in the future.

It's also worth noting that this would not have been an issue with cached mode. For this client, online mode was required.

Additional resources:

• https://social.technet.microsoft.com/Forums/office/en-US/0d45b1b0-3047-4666-ad04-217e98ed8823/slow-online-mode-browsing?forum=exchangesvrclients
• http://www.wardvissers.nl/tag/tcpackfrequency/
• https://communities.vmware.com/message/2439033

Update (Nov 2016)
There seems to be a general issue with VMware Tools version 9.4.10 (5.5u2) and later with Exchange Server. I've now run into this as a performance issue with various versions of Outlook and Exchange 2013/2016. What seems to be the best solution is downgrading the VMware Tools to version 9.4.5 (5.5u1).

Also notable is that it seems to be client dependent. It seems to be an issue with Windows 7 clients and either not at all or less so with Windows 8.1 or Windows 10 clients.