How to fix Microsoft Outlook running slow or freezing when opening large mailboxes

Summary

This guide helps you troubleshoot and resolve: How to fix Microsoft Outlook running slow or freezing when opening large mailboxes. Follow the steps below to fix the issue.

Common Causes

When Outlook struggles to open or respond, the cause is usually one of four things: a bloated cache, conflicting add-ins, a corrupted data file, or a mailbox that has grown beyond what Outlook can handle comfortably. The steps below walk you through each fix, starting with the quickest wins and moving towards more involved repairs. Work through them in order and test Outlook after each step before moving on.

Quick Fix Steps

If you only have a few minutes, try these in order. Most users see a noticeable improvement after the first two.

1. Restart Outlook in Safe Mode — Hold Ctrl, then double-click the Outlook shortcut. Choose Yes when prompted. Safe Mode loads Outlook without add-ins, so if it runs smoothly here, an add-in is the culprit.
2. Clear the Outlook cache — Close Outlook, then delete the contents of %localappdata%\Microsoft\Outlook\RoamCache.
3. Reduce the Mail to Keep Offline setting — File → Account Settings → Account Settings → double-click your account → under Mailbox, change Mail to keep offline to 6 months or 12 months.
4. Compact the OST/PST file — File → Info → Account Settings → double-click the account → click More Settings → Advanced tab → in the Outlook Data File section, click Compact Now.

Step 1: Disable Add-ins

Add-ins are the most common cause of a slow or freezing Outlook. Even trusted add-ins from reputable vendors can conflict with updates to Outlook or Windows.

1. In Outlook, click File → Options → Add-ins.
2. At the bottom of the window, next to Manage:, select COM Add-ins and click Go.
3. Untick every add-in except Microsoft Exchange Add-in and Microsoft Outlook Add-in, then click OK.
4. Restart Outlook and check performance.

If Outlook runs normally with add-ins disabled, re-enable them one at a time, restarting Outlook after each, until you find the one causing the problem. Once identified, check the vendor's website for an updated version, or remove it permanently.

Step 2: Clear the Outlook Cache

Outlook stores temporary data in a local cache to speed up performance, but over time this cache becomes stale or corrupted, which can cause freezing.

1. Close Outlook completely. Check Task Manager (right-click the taskbar) to confirm no Microsoft Outlook processes are running.
2. Press Windows + R, type %localappdata%\Microsoft\Outlook\RoamCache, and press Enter.
3. Select everything in the folder (Ctrl + A) and delete it.
4. Restart Outlook. The cache will rebuild automatically.

Step 3: Reduce Mailbox Size and Sync Settings

Outlook slows down significantly when the local OST or PST file exceeds 10–15 GB. If your mailbox is large, trimming what Outlook syncs locally is often the single biggest performance win.

Adjust the Offline Mail Slider

1. Click File → Account Settings → Account Settings.
2. Select your email account and click Change.
3. Drag the Mail to keep offline slider to 6 months or 12 months.
4. Click Next → OK → Finish, then restart Outlook.

Archive Old Mail

Move older items out of your main mailbox and into an archive to keep the active mailbox lean.

1. Click File → Tools → Mailbox Cleanup → Archive.
2. Choose Archive this folder and all subfolders, set a date (for example, items older than 12 months), and pick an archive location.
3. Click OK.

Step 4: Repair the Outlook Data File

If Outlook still freezes after the steps above, the underlying data file (OST or PST) may be corrupted. Outlook includes a built-in repair tool called ScanPST for PST files. OST files are typically repaired by recreating the profile or deleting the OST to force a resync from the server.

Repair a PST File (POP/IMAP Accounts)

1. Close Outlook.
2. Open File Explorer and navigate to C:\Program Files\Microsoft Office\root\Office16\ (the path may vary slightly depending on your Office version).
3. Double-click SCANPST.EXE.
4. Click Browse, select the affected PST file (usually in %userprofile%\Documents\Outlook Files\), and click Start.
5. If errors are found, click Repair.

Repair an OST File (Exchange / Microsoft 365 Accounts)

1. Close Outlook.
2. Locate the OST file in %userprofile%\AppData\Local\Microsoft\Outlook\.
3. Rename the file (for example, add .old to the end).
4. Restart Outlook. The OST will be rebuilt automatically by resyncing with the Exchange server.

Step 5: Check for Outlook and Windows Updates

Microsoft regularly releases fixes for known performance issues. Running an outdated version of Outlook is a common cause of freezing.

1. Open Outlook and click File → Office Account → Update Options → Update Now.
2. While you're there, click About Outlook and confirm you are running a current build (Microsoft 365 Apps for Enterprise is updated monthly).
3. Open Windows Settings → Windows Update and install any pending updates.
4. Restart your computer.

Step 6: Recreate the Outlook Profile

If none of the above has worked, the Outlook profile itself may be damaged. Creating a fresh profile is the last resort before reinstalling Office.

1. Open the Mail control panel: press Windows + R, type control /name Microsoft.Mail, and press Enter.
2. Click Show Profiles → Add.
3. Name the new profile (for example, "Outlook New") and follow the prompts to add your email account.
4. Once the new profile is created, return to the Show Profiles dialog and set When starting Outlook, use this profile to your new profile.
5. Click OK and start Outlook.

If performance improves with the new profile, you can migrate your data across and remove the old profile. If Outlook still freezes, the issue is likely with the Office installation itself, and a repair or reinstall of Microsoft 365 may be required.

Still Having Issues?