Why Outlook 2007 is slow: Microsoft’s official answer

A knowledgebase article published last week acknowledges performance problems with Outlook 2007, though it says these only occur with mailboxes larger than 2GB:

You may experience one or more of the following performance problems when you are working with items in a large Personal Folder file (.pst) or in a large Offline Folder file (.ost) in Microsoft Office Outlook 2007 … Note When you perform the same operations on the large .pst or .ost file in earlier versions of Outlook, the same performance problems do not occur. These problems may occur if the .pst or .ost file is larger than 2 GB. Additionally, the performance problems are more pronounced when the .pst or .ost file is larger than 4 GB.

I think this is optimistic and that smaller mailboxes are slower too; nevertheless, it does confirm that that the size of the local store is the key issue.

If you use Exchange, the local store is the .PST or .OST file on your workstation or laptop. If you do not use Exchange, a local .PST store is all you have.

Here’s what Microsoft says is the reason:

To accommodate new features, Outlook 2007 introduced a new data structure for .pst and .ost files. In this new data structure, the frequency of writing data to the hard disk increases as the number of items in the .pst or .ost files increases.

Intriguing, especially as I had thought the .pst format was the same in Outlook 2003 and 2007. The big change was from Outlook 2000 to Outlook 2003, when Unicode was introduced and the maximum size increased to 20GB.

I’d also like to know whether Microsoft is just stating the obvious here (bigger file, more disk access); or whether there is some exponential increase in disk writes, suggesting a design fault in the software. I have already noticed that if you show the I/O columns in Task Manager’s performance tab, Outlook 2007 shows some extraordinarily large numbers.

So what’s the fix? The news is not too good. In essence, you have to reduce the size of the local store. You can archive or move items to separate .pst files, or switch off cached mode so you always work online to Exchange.

The article doesn’t say it, but there are significant problems with switching off cached mode. These include hugely increased network traffic, problems with junk mail filtering, and loss of all your mail when using a laptop disconnected from the network.

The most imaginative suggestion is to filter the sychronization. For example, you could filter out messagse with large attachments, or all messages from last year or earlier. These messages will still exist in Exchange, but not in the local store.

Worth a try, but none of the workarounds is really satisfactory. Outlook 2003 worked fine with large mailboxes, Outlook 2007 does not. That’s a blunder.


158 thoughts on “Why Outlook 2007 is slow: Microsoft’s official answer”

  1. I found the problem to be my HOTMAIL account synced with my MS Outlook 2007. Once I removed it, outlook has been working perfectly. Once more MS has implemented something they did not fully test. Not a problem I don’t care for it anyway.

  2. Dear Tim

    I have SBS installed and one network one of the PC recieve emails in batches. Its downloads email all emails in one go. Like for one day it downloads 5 now 10 later and so no..
    Possibility you can email me on this.

  3. Hm, I do have some PST-Files which are bigger than 2GB and I don’t think it affects the speed of my outlook very strongly. Sometimes it takes a little longer to open an email but especially searching is really fast or do you know if third-party products integrated in Outlook (I work with the search tool lookeen) are not affected by that? Wouldn’t make any sense to me because they are using the same environment but you never know.

  4. My company every morning receiving email from outlook 2007 very slow until afternoon, Do anyone have a solution for this problem.

  5. One thing I am not getting in the whole article is “Outlook Version 2007 can have PST up to 20 GB approx” then how is the PST file size can be the reason for slow Outlook processing.

  6. I have read a many times that “Heavy PST/OST files make Outlook heavy indirectyl which leads to slow Outlook Processing”. Heavy PST file size is the most common factor behind Outlook speed.

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