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Performance problems with Microsoft Outlook 2007 .PST and .OST files

Microsoft Outlook 2007 suffers from serious performance problems when .PST and .OST files are used. Find out what the issue is and get the fix.

Microsoft Outlook 2007 suffers from performance problems when .PST and .OST files are used.

Opening new messages, moving between messages (even without the preview pane open), deleting or moving messages -- almost anything that involves accessing items in a .PST file -- eventually begins to exhibit terrible slowdowns in Outlook 2007.


The problem is not uniform. Some clients exhibit it far more than others, most likely due to user work habits, but all Outlook 2007 installations exhibit the problem to some degree.

The same issues also seem to appear when an .OST file is present, although the majority of problems have been reported by .PST file users.

Fortunately, Microsoft recently released a patch for Outlook 2007 (validation required) that changes the way Outlook 2007 deals with .OST and .PST files.

According to Microsoft Knowledge Base article 932086, You may experience performance problems when you are working with items in a large .PST file or in a large .OST file in Outlook 2007, part of the reason for the slowdown is a new internal structure for .PST and .OST files that increases the amount of disk access.

The patch optimizes the way writes are committed to .OST and .PST files. So my guess is that the optimization consists of causing the writes to happen in a "lazy," decoupled way -- i.e., writes to a .PST file are only handled after everything else in Microsoft Outlook has gone idle, and in a way that commits as many of them as possible at once.

Related resources on Microsoft Outlook 2007:

Watch out for a Microsoft Outlook 2007 indexing issue in Windows Vista

How to configure Microsoft Outlook 2007

How to use Microsoft Outlook 2007's Quick Parts feature

 Microsoft's original recommended workaround was to keep the default .PST size as small as possible -- for instance, by using the AutoArchive function to move out any messages older than a few months.

On my own system, I used a four-month threshold for archiving messages. As a result, I didn't experience as gross a slowdown as some other people. But even in my case, installing the Outlook 2007 patch showed a dramatic improvement in speed, so it comes highly recommended for all installations of Outlook 2007, regardless of the usage profile.

About the author: Serdar Yegulalp is editor of  Windows Insight, a newsletter devoted to hints, tips, tricks, news and goodies for all flavors of Windows users.


You should let people know that with Windows Update they'll get this patch automatically.
—Shawn C.


I installed the Outlook 2007 patch and it made negligible difference.

Here is what I have done to no avail:

  • Removed all non-essential plug ins.
  • Purged and compacted items in the .PST (I made the file 80% smaller).
  • Removed 75% of the accounts and moved them to Thunderbird.
  • Tried turning off virus scanning.
  • Removed 90% of my rules.

Still, it takes over a minute to receive six small emails versus 100-200 email per minute in Thunderbird.

I am a hunt-and-peck typist and Microsoft Outlook 2007 can't keep up! This is by far the worst version of Microsoft Outlook ever and I have been using it since it first came out. Plus, everything freezes when Outlook 2007 receives email and it just ignores mouse clicks.
—James G.


I suspect there may be a lot more wrong on the machine than just a Microsoft Outlook issue -- for instance, if the network or video card drivers need updating, that might manifest itself in this way. I know of at least one person who had painfully slow Outlook performance until a NIC driver update.
—Serdar Yegulalp, tip author


Where do the .OST and .PST files exist? If they are being manipulated by Microsoft Outlook, then they should be on the client machine. If so, then why do you say "after everything else in Exchange Server has gone idle"? That implies that these files are being read and written by the Exchange server -- an entirely different process, usually on a different machine.
—Alan P.


The .OST and .PST files are indeed kept on the client side and are written to by Outlook itself. My comment about Exchange was only meant to apply if you are using Exchange Server to receive email but are storing messages locally, so I do apologize if that was confusing. I've modified the tip to clarify this.
—Serdar Yegulalp, tip author

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