Troubleshooting Client Performance Issues

troubleshootingWhen you are an Exchange admin, you may find that one of the most problematic areas to troubleshoot is client performance. Inevitably, there will be a subset of users who have bad performance with Outlook-it’s slow, it stops responding, it errors-and these will be escalated to you. Knowing some of the common causes for poor client performance can help you to narrow down whether you have an actual technical issue, a misconfiguration, or a client behavior issue that needs to be addressed.

While not an exhaustive list, here’s some of the most common things that can cause poor client performance. We’ll talk about what they are, what symptoms to look for, and what you can do about them.


Cached versus Online Mode

Poor client performance often comes down to whether an Outlook 2010 or 2013 client is configured to run in cached mode or not. With cached mode, all mailbox content in 2010, and a configurable time period’s worth in 2013, is downloaded to the client and stored in a file called the OST. The Outlook client accesses all data within that OST, and syncs to Exchange happen in the background. When you are not using cached mode, all access to data has to go over the network. Browsing from one folder to the next, or moving an item from folder to another can both be slow, and searches can be painfully slow. If your Exchange server is remote, frequent disconnects can become noticeable during high-latency or reduced bandwidth situations.

Symptoms: overall slow performance, user receives notifications that their connection to Exchange has been lost, very long times to open attachments, slow performance on numerous actions.

Fix: Access the properties of the Exchange account in Outlook, and check the box to use Cached mode. Restart Outlook for this setting to take effect. In Outlook 2013, you can use the slider bar to set how much data is cached, from one month out to all.

Third-party plug-ins

There’s a ton of plug-ins for Outlook, from social media to mass communications to search and more. While many of these prove invaluable aids to their users, sometimes they can cause Outlook to grind to a halt.

Symptoms: Outlook takes a very long time to load. Switching from one item to the next can also take a long time.

Fix: Launch Outlook using the /safe switch to start without any plug-ins. If that resolves the issue, go into Outlook’s plug-ins and disable them all, then enable one at a time and restart Outlook each time until you find the one that causes the problems. Consider updating to the latest version before you just rip it out, as often these plug-in issues are resolved by simply keeping them up to date.

Proxy servers

Proxy servers controlling Internet access can also be a problem, particularly for users who are remote to their Exchange server. When Outlook uses a proxy because the operating system is configured to do so, you will have fewer problems then when a transparent proxy is in place. You can check the connection settings in Control Panel, or you may have to consult the security or networking teams.

Symptom: intermittent slowness, especially at peak Internet access times even when bandwidth utilization is low.

Fix: White-list Exchange FQDNs to permit clients to access without going through the proxy.

NAT pool depletion

If you have users at a remote location who access Exchange over the Internet using OA, you may find that the number of connections Outlook clients make can exhaust available ports for NAT.

Symptom: Intermittent failures, especially when looking up Free/Busy or GAL information. Outlook connectivity will show one or two established connections plus many more that are either attempting to connect or fail.

Fix: Add an additional ip.addr to the NAT pool, or expand the range of ports available for xlates.

Deep Packet Inspection

This impacts remote users more often than users in HQ. DPI is a security feature on many home and SOHO routers designed to enhance security.

Symptom: Users may find very slow performance from Outlook, both at launch and during all activities. Outlook may also stop responding for a moment as you call up the GAL or move from one folder to the next.

Fix: Disable DPI.

Global Catalog queries

Outlook is dependent upon Global Catalogs for GAL lookups. When a user is in a location without a GC, these lookups may be serviced by any GC in the forest.

Symptom: GAL lookups take a very long time to resolve.

Fix: Global Catalog servers should be in every site where users connect. Enable the GC role on an existing DC, or deploy a new GC to sites with multiple users. Ensure that there are enough GCs in sites where users connect over the Internet or VPN to access Exchange.

Folder/object limits

Outlook has maximum recommended numbers for both total folders in a mailbox, maximum depth of subfolders, and total number of objects within a folder. Exceed them, and Outlook performance is bound to suffer. Try to keep the number of objects (messages or other folders) in any single folder below 5000 to maintain best performance.

Symptom: Slow response from any action in Outlook, but especially when changing folders, sorting, or searching for items.

Fix: Reduce the number of folders/objects in any single folder by moving/sorting content.

Too many mailboxes

This frequently hits administrative assistants and users who work with multiple shared mailboxes. The more mailboxes you have open in Outlook, the more connections Outlook must make and the number of network operations Outlook must perform.

Cause: Slow startup times and closing times for Outlook.

Fix: Dismount mailboxes and/or calendars that are not required. Configure shared mailboxes to run in online mode only.


Readers of the blog know how bad PSTs can be, so this should come as no surprise. When Outlook has PSTs mounted, all access to those PSTs is handled serially. Outlook is not going to do anything else while trying to access a PST. When you have PSTs mounted over the network (which is explicitly unsupported by Microsoft) not only can this impact Outlook, but it can cause performance problems on the server for all users-even those without a mailbox.

Cause: PSTs

Fix: Get rid of them. Since that is easier said than done, start with reducing the need for them by increasing mailbox size, consider moving to an archive mailbox, close PSTs that are not required to be open, and move any network PSTs to the local hard drive.

When it comes to troubleshooting Outlook performance, the above are some of the most frequently encountered causes of performance issues. Address them, and your user should be up and running with great performance in no time.

Written by Casper Manes

I currently work as a Senior Messaging Consultant for one of the premier consulting firms in the world, I cut my teeth on Exchange 5.0, and have worked with every version of Microsoft’s awesome email package since then, as well as MHS, Sendmail, and MailEnable systems. I've written dozens of articles on behalf of my past employers, their partners, and others, and I finally decided to embrace blogging and social media, so please follow me on Twitter @caspermanes if you enjoy my posts.


  1. David Black · June 28, 2013

    Great list – a really useful one when you get to troubleshoot client issues on Outlook. In addition to all the reasons that are related to the network infrastructure or Outlook itself, I would add a very prosaic one – malware. I remember many cases when the antimalware program has failed and as a result computer performance was really slow. While some leaner apps didn’t feel it that bad, with the always resource-hungry Outlook it was a nightmare.

  2. Lourdes · June 29, 2013

    Why can’t we have a simpler design for our e-mail platforms? One of the reasons why Gmail is gaining so much popularity is because it’s simple and truly user friendly. I don’t think Outlook needs to install plenty of these plug-ins like social media. Keep things neat and precise.

  3. Kirsten · June 30, 2013

    One of the recommendations I can make is to always keep the Outlook mailbox clean and clear. That means if the messages are worth keeping, ditch them. This way, you don’t have to create different folders and subfolders. Second, avoid signing up to mailing lists that you don’t FULLY love.

  4. Casper Manes · July 23, 2013

    @david-thanks, glad you liked it!

    @Lourdes-it’s not the platform in this case, it’s the client. Try Outlook Web App and I bet you will love it!

    @Kirsten-absolutely, great tips!

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