10 reasons why Outlook is running sooooo slowly

Are your users on Microsoft Outlook and Exchange server  complaining about lethargic performance? Here are some things to troubleshoot when you want to quell the griping.

  1. snail 300 editAntiquated Software. Are you still on Exchange 2003? That old 32-bit warhorse will have a tough time meeting the email demands of a modern organization. New 64-bit versions of Exchange–2007 and 2010–support more memory and bigger buffers, as well as other speed enhancements. As a result, they can be from five to 10 times more efficient than 2003 in handling mail. While upgrading to a new version of Exchange isn’t an immediate solution to your problems, it’s something to advocate as a long term solution.
  2. Mailbox Limits. Both Exchange 2003 and 2007 have 2GB limits on mailbox sizes. However, Outlook users can  exceed those limits. The rub is, the greater that 2GB limit is exceeded, the bigger the hit the user will see in performance. One way to address that problem is to deploy an archiving solution. It will automatically archive a user’s emails when his or her mailbox approaches or exceeds the 2GB limit.
  3. Overstuffed Folders. Too many messages stored in a single folder will impair Outlook’s performance. Microsoft recommends that between 3500 to 5000 messages should be placed in a single folder. An archiving solution can address this folder problem, too, as well as creating more top-level folders or sub-folders in folders with high growth rates such as Inbox, Sent and Calendar.
  4. Anti-Virus Software. Local anti-virus software can make Outlook work harder than necessary. Each time a message is opened, its body and any attachments to it are scanned by the anti-virus software. That can result in delays of as much as 20 seconds. An alternative to local virus scans is to scan messages in transit at the Exchange Hub Transport servers. With malware scans performed at the servers, you can disable scanning at the client end of things and boost performance for your users.
  5. Flakey Indexing. Indexing was introduced into Outlook in 2007. It’s a great feature because it quickens the time it takes to find anything in the program. If the index  is corrupted, however, the function can cripple Outlook. How do you know if the index is corrupt? If Outlook never seems to finish indexing messages, that’s definitely a sign that something is wrong with the feature. The best thing to do then is to delete the index and re-index all your messages. Yes, that’s going to slow things down in Outlook for a while, but faster performance will be the reward at the end of the process.
  6. Too Many Rules. Users create rules in Outlook to manipulate messages in the program. Messages from the boss, for instance, might be shipped to an Ignore Until Harassed folder. The amount of space allocated to each folder for rules is 32KB, which will accommodate 40 to 50 rules. As a folder approaches the space ceiling, Outlook starts to slow down. The solution to this problem is simple: use fewer rules in your folders.
  7. The Myth of New Hardware. A brand new desktop will run faster than a four-year-old clunker, but you should be careful that new hardware isn’t masking old problems. A user with an old machine and 80,000 messages in his or her inbox may see a performance boost with a new machine, but that doesn’t remove the fact that there were too many messages in the inbox in the first place. What’s more, while the user with the new computer may be whistling along at his or her desk, those 80,000 messages are still taxing Exchange’s performance and in turn, hurting other users on the system.
  8. Shared Folders Headache. Outlook 2007 and 2010 turn on by default shared folder caching. That can create performance problems, especially for Outlook’s calendar module. You see, with shared folder caching on, when you open another person’s calendar, the entire calendar is sent to your calendar. What’s more,  when you’re online, Outlook syncs your calendar with all the calendars you’re linked to on your system. That can really slow things down. It can take up to 20 seconds before your calendar is displayed. The simple solution to that problem is to turn off shared folder caching.
  9. Wacky MAPI. Most of the devices in Outlook/Exchange environments use MAPI to communicate. That can create bottlenecks for both clients and servers. On a client, for example, you may have Outlook, Microsoft CRM, integrated voice mail and an email archiving application all trying to send and receive information on a single network adapter. On the server side, you have numerous requests being sent  to your server from numerous devices deployed by numerous users. One way to address those problems is to scale your Outlook/Exchange environment on the basis of the number of devices on the network, not the number of users. In addition, efforts should be made to control the number of MAPI devices in use on the system.
  10. Disappearing DNS. Although it’s not a common problem, sometimes a user is sent to a phantom server by a corrupt DNS. When Outlook can’t find the phantom server, it has to search for another. That results in delays that users see as slow performance. Running Exchange’s “Best Practice Analyzer” will most often identify those kinds of DNS problems.
Written by John P Mello Jr

John Mello is a freelance writer who has written about business and technical subjects for more than 25 years. He is frequent contributor to the ECT News Network and his work has appeared in a number of periodicals, including Byte magazine, PC World, Computerworld, CIO magazine and the Boston Globe


  1. denny chow · September 6, 2010

    Switching e-mail platforms may probably be one of the more excruciating IT tasks, especially when it comes to a large scale company like ours. After running into some major concerns with Outlook, a good number of our company’s staff have switched to browser based e-mails as a back-up.

    What happened, after quite an alarming stretch of time, is that some teams were sending to double the addresses they needed to. Everything was sent to their office address, then CC’d to their back-up e-mail.

  2. oliver sax · September 10, 2010

    Although a lot of companies still use Outlook Express for its cross-platform functionality, I’ve personally been very displeased with the application’s many versions over the past few years. The 2GB mail limit might have been a good size five years, but it definitely isn’t doing anyone any favors right now. Do any of you have any alternative applications you can recommend instead of Outlook Express? I’d love to hear about them.

  3. Tricia Lace · September 10, 2010

    I think the ideas behind a lot of these complications had really good intentions behind them. It’s just unfortunate that Outlook Express didn’t have an efficient way of utilizing its memory or integrating any fail-safes in case anything went wrong. Indexing is a great feature, but the main index just seemed too detached from the rest of the program. It’s always a bother to delete it and start from scratch to hopefully improve future performance.

  4. Keanan · September 10, 2010

    The integration of Rules in Outlook Express was actually a great feature on paper (I find it funny that I’m referring to it in past tense). The rules feature offered a great amount of versatility to the program, offering the ability to customize a lot of the functions Express already had on hand. It does begin to bog down after a lot of the rules have been coded, but hopefully in later versions they can work around the memory restraints.

  5. Maurice Volaski · October 28, 2010

    20 seconds is an eternity to computers that run in gigahertz.

    Where is your evidence that antivirus software slows down outlook up to 20 seconds? Which antivirus software? What kinds of messages? What are other variables, such as machine speed, RAM and load?

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