Outlook slow performance and hanging

Copper Contributor



We have Exchange 2016 CU23 and late last year we encountered the above symptoms with outlook 2016 and after a lot of back and forth with support, it turned out to be a bottleneck with NTLM authentication requests on AD servers. We increased maxconcurrentAPIs to 50 and this helped us for a while.


Last week we began to encounter the same symptoms again but this time we have outlook 2021 installed, and this could be the only variable in the environment other than regular windows and exchange security updates.


Immediately we thought it may be the same root cause as last year, we noticed some semaphore holders spiking towards the maximum so we increased the limit again to 80 but this time, we continued to experience the same issue without hitting the new maximum for the maxconncurrentapi.


We have 5000+ users and almost 1500 out of them are working on VDI with online mode enabled while the rest are working on physical machines on cached mode.


Usually, the VDI users are the ones who are experiencing the slowness and freezing the most. Since there was no much changes other than outlook 2021, we rolled back a group of VDI users to outlook 2016 but they still encountered the same issues.


Support is engaged and they collected ETL files for analysis but by experience, sometimes this could lead to incorrect findings as happened last year where ETL files were pointing towards a network issue and the issue turned out to be due to NTLM authentication bottleneck.


OWA is working fine at all times and server’s performance during the issue is perfect with no over utilisation of memory/CPU/disk IO. Also no weird events during the issue that we can see in event viewer.


We have a DAG with 12 servers, 8 in main site and 4 in DR site.


I’m writing here to explore if the above symptoms (or similar) happened with any of you and how did you handle it since, frankly speaking, we have been stuck in this issue for long now.


Any help or hints would be appreciated.


3 Replies

@Max396 Hello did find the solution we are experiencing the same issue ?

@MhNashiriS hello, actually the slowness/hanging of outlook are symptoms of many causes.


You need to dig deeper into the following to try to identify the cause:-

  • Outlook connectivity from end user perspective (using resource monitor to see outlook.exe connections and latency, may be use fiddler or wireshark)
  • make sure there are no authentication bottlenecks with AD, by default MaxconcurrentAPI limit is 20 on exchange and AD, you can check performance monitor and check semaphore holders and semaphore waiters during the issue, if the numbers are close to 20, it might be worth it to try to increase MaxconcuurentAPI and restart netlogon service, check this link before your proceed.
  • Try to further isolate the issue by minimizing the scope of the users who are seeing the issue, for example all users having same outlook version, users on specific servers or specific DAG, users with specific network connectivity, etc.
  • If you have users working with online mode, try to use cached mode for a subset of the users and see if this will help.
  • check for high mail queues during the issue specially if you see “outlook is trying to connect” message on outlook.
  • if you have users with online archive hosted on exchange, try to disconnect the archive for some users for testing purposes and see if it will help.
  • try any of the above one by one to identify which of them will help you.

finally, please let me know when you resolve the issue specially if you find it to be caused by another cause not listed above.


Good luck 

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