I'm currently running outlook 2010 and I have a .pst file of about 25 gigs in size.

My email is currently being imported via POP3 so I am not running any IMAP or keeping a copy of email onto the server in POP3 mode.

I seem to be having more and more issues every day with the performance of outlook. At times it will freeze for 15-20 seconds while opening a new email or simply clicking reply.

I have a fairly new computer with a i7 processor. Would anyone have any suggestions on what would be causing this "freeze" and/or ways to trouble shoot to find a solution?

  • 1
    Reduce the size of that .pst seems like an obvious thing to test - and its easy. Create a new one, copy over, say the last 100 messages and disconnect the big one. See whether performance improves. – uSlackr Jun 13 '12 at 21:38

Seeing as the limit of a PST file is 20GB, and yours is 25GB I can see that definitely being an issue. It's a known issue in all versions of Outlook that anything over 10GB you will start seeing performance issues.

You can export data to another PST file to reduce the size: http://support.microsoft.com/kb/982577

  • Seems like the limit isn't 20GB ... Read it's 50, and that you can change it as well... – Noctis Aug 25 '15 at 10:57

It is a well known problem with Outlook, and it can also lead to a corrupt PST eventually. Your best bet is to create a new PST. You can do this while still retaining the emails in the old one. This guide contains pretty much everything you need to know.


Archive, archive, archive.

If you don't want to allow Outlook to do this automatically at regular intervals (default: 14 days, can be set to daily) then create a new PST and use Outlook's manual archiving capability. In 2010, Click File - Cleanup Tools - Archive. Select the 2nd option ("Archive this folder and all subfolders") then select your current personal folders file as the "source"; choose a convenient date for "Archive items older than" and then select your newly created file as the "Archive file" and hit [OK]. Watch the bottom right-hand border of Outlook's window for progress notes - it tells you which folder it is currently engaged in archiving. Note this can take a while! A long, long while, I think, with a 25 GB file.

Once the process is complete, the PST file is still the same size it was before and has to be compacted. It might be a good idea to back it up first though.

Still in Outlook, go to File - Account Settings - Account Settings again - select the Data Files tab. Highlight your main (default) PST file, and click the button for "Open Folder". This opens a File Explorer window in the relevant folder, and you now need to CLOSE OUTLOOK or the next step won't work.

Right-click your large PST file and select "Copy", then right-click anywhere in the white space in the same folder and click "Paste". This will create a copy of the file, named "{same name as original} - Copy.pst". Again, not a lightning-fast process, but depending on your computer and disk drive speed shouldn't take more than a minute or so.

LEAVE OUTLOOK CLOSED. Open your Control Panel and type MAIL. This will show you a CP applet called Mail or Mail (32-bit) - click to open it and select Data Files - you will notice this is the same dialog box you were using just now from within Outlook. Once again highlight your large PST file and click the "Settings" button. Click [Compact Now] and walk away. Go make coffee, watch a movie on TV, have a bath, go to bed, have a good night's sleep, eat breakfast; if you're lucky it will be done. Maybe. Seriously.

I recommend this procedure (doing it via the CP applet rather than from within Outlook) because if OL is open it continues to try to send/receive mails at whatver predetermined interval you have set. At best this is going to slow down the process, but if you are already experiencing issues with send and receive it strikes me it would be best to avoid that while working on the file.

Good luck :-)


Other simple way is to delete some old messages. In your case, it will be a lot of emails and then compact the outlook pst file. Another option is to create another pst files and copy all the items from a particular date; lets say prior to 1 Jan 2013 to that file and then compact your current outlook file. See how it works. Compact a file: File-Account Settings-Account Settings-Data Files-Settings-Compact Now. This will take several minutes once and safe several more afterwards :)

  • The author doesn't want to delete old emails. The very fact he has 25GB of emails means he is keeping ALL the emails for a reason. – Ramhound Feb 20 '13 at 14:08

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