Outlook in Online mode is slow connecting to Exchange 2013 – A blog post from “Macster” and information from “Festivalman”, thanks guys !



Thanks Macster and Festivalman for this tip !


Before applying this TCP Ack solution, the below conditions must be met:

- OWA connection and mail browsing is very fine, whereas an Outlook online mode (i.e. not cached mode) connectivity is quite sluggish when mailboxes are on Exchange 2013…

- If OWA is slow as well, then the issue may be a general network slowness issue – check the network latency using Ping or PsPing (Advanced ping tool from Windows SysInternals)

- On Outlook Connection Status dialog box (CTRL+Right Click the Outlook icon on the Windows notifications part of the taskbar), Avg. Proc. time is fine, below 50~60ms, and Avg. Resp. time is over 110ms.



Cause seems that it looks like it’s caused by the 200ms timeout setting on ACKs, probably causing more TCP retransmits – using Netmon and an Outlook connecting to an Exchange 2013 lab infrastructure may confirm that point …

To summarize Macster and Festivalman findings, the solution is to create a TcpAckFrequency registry key and set it to 1.



My recommendation is not to change this registry key on the desktop(s) until you confirmed the behavior using a Network Trace, and until you are falling Under the above mentionned conditions (especially OWA browsing is fine, Outlook online browsing is slow)


More information about the TcpAckFrequency registry key:

Quoting from https://support2.microsoft.com/kb/328890:

- TcpAckFrequency is a registry entry that determines the number of TCP acknowledgments (ACKs) that will be outstanding before the delayed ACK timer is ignored.

- TCP uses delayed acknowledgments to reduce the number of packets that are sent on the media (Wifi, Wire,…)

- As data is received by TCP on a particular connection, it sends an acknowledgment back only if one of the following conditions is true:

  • No acknowledgment was sent for the previous segment received.
  • A segment is received, but no other segment arrives within 200 milliseconds for that connection.

Typically, an acknowledgment is sent for every other TCP segment that is received on a connection unless the delayed ACK timer (200 milliseconds) expires.

- You can adjust the delayed ACK timer by editing the following registry entry.


HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesTcpipParametersInterfaces<Interface GUID>



Value Type: REG_DWORD, number
Valid Range: 0-255
Default: 2
Description: Specifies the number of ACKs that will be outstanding before the delayed ACK timer is ignored. Microsoft does not recommend changing the default value without careful study of the environment.



More explanations and complete solution:

Outlook slow after migrating to Exchange 2013


Slow online mode "browsing"


Again, to give back to Caesar what belong to Caesar, thanks to Macster and Festivalman for sharing this finding ! That will save trouble for many people out there !