Comprehensive Guide on Addressing Slow / Sluggish Outlook Performance to Microsoft Exchange

Identifying and Solving Performance Issues with Microsoft Outlook and Exchange

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Solution:  To check to see if indexing may be a performance problem, check the Indexing Status in Outlook, see if Outlook is indictating that Indexing is being performed.  While indexing is being performed, expect system performance to be degraded.  You may find that the system never finishes indexing messages (, that typically indicates that the index is corrupt, however while an index is corrupt, the Indexing service is still attempting to index Outlook, so not only is your index not getting updated, but the system is chewing up system cycles to continuously index the system and never completes.  In the case where Indexing never seems to be completing, delete the index and choose to reindex the entire mailbox.  This may take a long time with a large mailbox, and during that time system performance for the user will be degraded, but once the indexing has been completed, performance will be improved.

Too Many Folder Rules / Inbox Rules

A not so common problem but one worthy to not are users that have a lot of Inbox or Folder "Rules" in Outlook experience slowdown of Outlook.  Outlook has a limited (32kb) memory space allocated for rules.  Users that add in too many rules not only have Outlook slow down because the rules have to fire off and process the rules before the user actually sees the message in their Inbox, but also having too many rules causes Outlook to slow down in general.  This KB article details the issue:   Effectively, there is a 32-kilobyte (KB) "packed data" memory limit for the rules on each folder in the Inbox or in a public folder, each inbox rule takes about 660 bytes, therefore the maximum number of rules is 40-50 rules per folder, and the more rules the user has, the slower performance they will receive in Outlook.

Solution:  Evaluate the rules in use and try to minimize the number of rules being used.

Local Workstation Performance

A user's workstation can impact performance of Outlook, so a 10 year old system with 512m of RAM and a lot of applications will run slower than a brand new i7 processor system with 4gb of RAM and only a handful of applications.  However workstation performance frequently masks real problems.  A user who had a slow system with 50,000 messages in their Inbox that gets a faster system (and still has too many messages in their inbox) may experience acceptable performance after the system upgrade, but the inherent problem of having too many messages still exists.  In many cases, the faster system will appease the user as their personal performance problem will seem to have gone away, however their impact on the Exchange Server (having to deal with a system with too many messages in a folder) will continue to impact the performance of the Exchange server and thus the performance of other users.  Additionally, disk defragmentation is also something to consider.  A fragmented hard disk with a user who has a large OST can find that the access to the Cachemode file is a source of performance problems.

Solution:  Get a faster system for the user and defragment the user's hard drive using a tool like Microsoft's Sysinternals to get faster client system speed, however make sure that all other issues like mailbox size, number of messages in a folder, number of inbox rules, and the like have all been addressed, otherwise inherent performance problems will still exist.

Caching Shared Folders

A nasty default setting in Outlook 2007 and Outlook 2010 when a user is running Cachemode for Outlook is a setting that caches "Shared Folders".  It is selected by default and when a user opens another person's calendar, it downloads the other person's entire calendar so that it does a Cachemode on other user's calendars.  This is great if you work offline and want to potentially look at someone else's calendar, however when a user is online, each and every time another calendar is open, Outlook goes out to the other user's calendar, checks for changes, and does a complete calendar synchronization before the user's calendar is visible.  This is frequently reported as "slow performance for calendar access" as it can be 5, 10, 20 seconds before a user's calendar appointments are visible.

Solution:  Turn off the caching of Shared Folders by going into the Outlook profile (File / Info / Account Settings in Outlook 2010; or Tools / Options / Account Settings in Outlook 2003 or 2007), click on the Advanced Tab, and uncheck the Download Shared Folders checkbox as shown in the graphic below.

Limitations of MAPI

Pretty much everything in the Exchange/Outlook world uses MAPI as the method of communications (other than Apple Macs that are now using Exchange Web Services (EWS), or Outlook Web Access that is also now using EWS)), but everything else (Outlook, Outlook Anywhere, Blackberry, most email archiving solutions, most integrated voicemail solutions, most faxing solutions) all use MAPI.  When too many things are using MAPI, there is a bottleneck on the server or the client.  For the client system that has Outlook, Microsoft CRM, integrated voicemail to Exchange, email Archiving, etc, it is possible for all of those applications to choke the performance of a client system, so despite having a super high CPU performing system with a lot of RAM, there's only a single network adapter that all MAPI communications is sequentially trying to send and receive information.  On the server side, the server is receiving MAPI requests from dozens / hundreds / thousands of users each with potentially dozens / hundreds of MAPI requests.  Add on to that the Blackberry Service where each Blackberry device generates over 3.6 IOPS (, which is equivalent to adding 4 users to a server, an organization with 500 users, 250 iPhones and Windows Mobile devices, and 125 Blackberries is really hosting over 1,250 "users" from a load perspective, not just 500 "users".

Solution:  Scale your environment to take in account not just the number of "users" you have, but the number of "devices" (and the performance impact of each of those devices) which considering most organizations now have users with mobile phones and the MAPI impact of Blackberries, servers are frequently not designed to handle the true load the network.  Minimize the number of MAPI applications in use, many organizations are getting rid of their Blackberry Enterprise System (BES) servers and adding ActiveSync applets to the Blackberry devices (products from companies like NotifySync, AstraSync, or the like) making all mobile devices (including Blackberries) communicate over native ActiveSync.  Scaling Exchange and managing MAPI devices and applications can help to improve overall Outlook/Exchange performance.

DNS and DSACCESS Problems

There have been isolated pockets of problems where a network DNS problem or DSACCESS problem causes client performance access.  Effectively, it has been seen where DNS is pointing a user to access the Global Catalog for a domain controller that no longer exists and/or the IP address has been assigned to another (non-DNS) server, so when Outlook goes out to query the Global Catalog, Outlook is looking at a non-existent server and has to fail look-up and do a lookup to another server to access the directory (a 5-10 second delay on look-up of user names or in sending messages).  On the server side, this type of DNS problem also affects DSACCESS which is how the Exchange server does a look-up of user names so that when the Exchange server receives an email, it knows where to send the message.  More details on DNS and DSACCESS are referred to in the following KB article:

Solution:  Make sure DNS and DSACCESS are working properly, which running the Microsoft Exchange "Best Practice Analyzer" will typically identify problems with Exchange, DNS, and the like.  Key is to make sure that after running the BPA, if any errors occur, to fix the errors.  I have seen many organizations where errors are ignored because the IT or Exchange administrator is "certain" that a specific problem is not a problem when BPA clearly identified the issue as a problem and when we went in and fixed the noted BPA error, the Exchange problem went away.  Remember, Exchange needs to have a Global Catalog server nearby (meaning on the same Ethernet segment) so if your Exchange server is far from a Global Catalog Server, make sure that a Global Catalog server is added close to the Exchange server.

Server Hardware / LAN-WAN Performance Issues

It is possible for Outlook performance issues to be related to server hardware, LAN, or WAN performance issues, however unless an organization is still running Exchange 2003 on old hardware, most organizations with Exchange 2007 and Exchange 2010 have relatively decent new 64-bit hardware as well as have tested LAN and WAN for performance and have come up with limited issues with the infrastructure.  A quick test is to run the problemsome (slow / sluggish) user access off hours, see if the problem persists at 9pm as it does 9am, and if the problem persists even at a time when the servers / LAN / WAN have limited use, then it's likely not a Server / LAN / WAN problem.  Common tests can be run on servers (perfmon) to check on the server utilization of an Exchange server.  Additional LAN/WAN performance tests can be run to make sure the LAN and the WAN are performing efficiently.   Frequently, running Outlook Web Access on a user's workstation to test performance can be a measure of whether a problem is specific to Outlook or to the LAN/WAN/Server.  If OWA runs very fast on the same workstation the user complains that Outlook runs slowly against Exchange, then likely the problem is not Server / LAN / WAN related, but like something related to Outlook, # of messages the user has in folders, size of the user's mailbox, or the like.  Other server problems include things like an anti-malware product not properly configured on a server.  If the anti-malware is not set to EXCLUDE scanning of the Exchange EDB database and LOG files, the anti-malware software will sit all day chugging away trying to scan the Exchange database and logs, taking up 10, 20, 30% of the server utilization to scan something it doesn't have the ability to scan.  Also I've seen tape backup systems running in the middle of the day trying to backup Exchange because the backup is taking more than a night to complete a backup and thus system performance is drastically degraded while Exchange is being attempted to be backed up.  These are all common server / LAN / WAN side issues that need to be checked to make sure these are not contributing factors to "slowness" of Outlook.

ActiveSync Consumption Causing Exchange Connection Problems

{Added 5/11/2011}  Microsoft has posted a knowledgebase article that addresses a problem where Exchange runs out of resources (caused by ActiveSync overload) that prevents other Exchange users (both ActiveSync as well as OWA/Outlook Anywhere users) from connecting.


Frequently Asked Questions

Q:  My mailbox has always had a LOT of messages in the INBOX and I've always had a really large >2GB mailbox, so why all of a sudden my performance has slowed down, it must be a Server / Network problem, not me, right?

A:  Liken it to the straw that broke the camel's back, as much as your INBOX and Outlook usage has always been excessive, the issue is that today, your usage is now even more excessive than it was before (so you have added more straws to the camel's back) and likely you are no longer the only person that has added straws to the camel's back as now dozens if not hundreds of users now have tons of messages in their Inbox, Calendar, Sent Mail folders, really large >2GB mailboxes, the addition of mobile phones (which effectively 7 years ago only a handful of execs had Blackberries, now "everyone" practically has a mobile phone in addition to their desktop/laptop), in addition mailboxes are getting larger and larger, in addition things like McAfee bugs or iPhone bugs all in combination push Exchange to the point where it slows down.  So it is the combination of all these stresses on Exchange that has caused a noticeable slowdown with Exchange and thus Outlook performance issues.

Q:  If Exchange is slowing down because of usage, can't we just add more Exchange servers and get faster servers to make the problem go away?

A:  Yeah, kind of, you can kick the can down the road by adding more servers, faster workstations, etc, but it's the same conversation I have with my doctor every 6 months about my cholesterol, that "can't I continue to eat a fatty, rich diet and just take more cholesterol medicine," with that long pause, his closing his eyes, shaking his head, and the final response of "yeah, you can do that, you'll just prolonge when you'll have a stroke and heart attack by a couple years."  Same with Exchange, you have to address the cause of the problem, change behaviors and habits, not just kick the problem down the road

Q:  Can't Microsoft fix Outlook and fix Exchange so that we can have larger mailboxes and more messages in each folder?

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