Outlook Running Slow: How to Fix It

If Microsoft Outlook has started crawling along — taking forever to open, freezing when you search, or lagging every time you switch folders — you are not alone. Outlook is one of the most resource-intensive desktop apps in the Microsoft 365 suite, and it tends to get slower over time as your mailbox grows and your system accumulates cruft. The good news is that most performance problems are fixable without reinstalling anything.

Why Does Outlook Run Slow?

Before jumping into fixes, it helps to understand what actually causes the slowdown. Outlook performance problems usually come from one or more of the following:

• Large PST or OST files — Outlook stores your emails locally in a data file. When this file grows to several gigabytes, every operation takes longer.
• Too many add-ins — Third-party plugins like CRM connectors, PDF tools, and antivirus scanners all run inside Outlook and can significantly drag on startup and send/receive times.
• A corrupt or bloated search index — Outlook relies on Windows Search to power its search function. When this index is damaged or out of date, Outlook search stops working properly and puts extra load on the app.
• Syncing too much email offline — If your account is set to keep years of email cached locally, Outlook has to manage a huge dataset every time it syncs.
• Ageing hardware — A spinning hard drive or insufficient RAM will bottleneck Outlook regardless of any software fix you apply.

Work through the fixes below in order. Most users find their Outlook performance improves dramatically after the first two or three steps.

Fix 1: Disable Unnecessary Add-ins

Add-ins are the single most common cause of a slow Outlook startup. Each one loads when Outlook opens, and some are poorly written or simply no longer needed. Disabling the ones you do not actively use is the fastest win available.

1. Open Outlook and go to File > Options > Add-ins.
2. At the bottom of the page, make sure COM Add-ins is selected in the Manage dropdown and click Go.
3. Uncheck any add-in you do not recognise or no longer use. Common culprits include antivirus mail scanners, old CRM plugins, and Adobe or Acrobat connectors.
4. Click OK and restart Outlook.

If Outlook starts noticeably faster, you have found your culprit. Re-enable add-ins one at a time to identify the specific offender if you need to narrow it down.

Fix 2: Compact the PST or OST Data File

When you delete emails, Outlook does not immediately reclaim that disk space — the data file retains the empty space until you compact it. Over time this can add hundreds of megabytes of wasted space that still has to be read from disk.

1. Go to File > Account Settings > Account Settings.
2. Click the Data Files tab.
3. Select your data file and click Settings.
4. Click Compact Now and wait for the process to complete.

This process can take several minutes if your data file is large. Do not close Outlook while it runs. If you have recently imported a PST file into Outlook, compacting afterwards is especially worthwhile.

Fix 3: Reduce the Mailbox Sync Window

By default, Microsoft 365 and Exchange accounts may sync all your email to a local cache. If you have years of messages, this creates an enormous OST file. Limiting the sync window to the last 3, 6, or 12 months reduces the size of that local cache substantially.

1. Go to File > Account Settings > Account Settings.
2. On the Email tab, double-click your Exchange or Microsoft 365 account.
3. Under Offline Settings, drag the slider to reduce the cached time range — try 3 months or 6 months as a starting point.
4. Click Next and then Done. Outlook will resync the reduced cache.

Older emails remain on the server and are still searchable — they just will not be stored locally on your machine.

Fix 4: Rebuild the Search Index

A damaged Windows Search index causes Outlook to slow down every time you search, and can also cause incomplete or missing results. Rebuilding it from scratch forces Windows to re-index all your Outlook data cleanly.

1. Go to File > Options > Search.
2. Click Indexing Options.
3. Click Advanced, then under Troubleshooting click Rebuild.
4. Click OK to confirm. The rebuild runs in the background and may take an hour or more depending on mailbox size.

Search performance and speed should improve once indexing completes. For a full walkthrough of search-related issues, see our guide on fixing Outlook search when it stops working.

Fix 5: Archive or Delete Old Emails

The simplest long-term fix is also the most underused one — keep your mailbox lean. The more email Outlook has to manage, the slower it runs. Archiving old messages to a separate PST file, or simply deleting what you no longer need, reduces the active working set Outlook deals with on every sync.

In Outlook, go to File > Tools > Clean Up Old Items (sometimes labelled Archive) to use the built-in auto-archive wizard. You can set rules to move items older than a certain date to an archive file automatically. Empty your Deleted Items and Junk Email folders regularly too — they count towards your mailbox size even if you never look at them.

Fix 6: Disable Hardware Graphics Acceleration

Outlook uses GPU acceleration to render its interface, but on systems with older or poorly-supported graphics drivers this feature causes more problems than it solves — including lag, visual glitches, and freezing when switching between emails.

1. Go to File > Options > Advanced.
2. Scroll down to the Display section.
3. Check the box for Disable hardware graphics acceleration.
4. Click OK and restart Outlook.

This is particularly effective on laptops with integrated graphics, remote desktop sessions, or any machine running older display drivers.

Fix 7: Run Office Repair

If Outlook is slow due to corrupted Office installation files, the built-in repair tool can fix it without needing to uninstall and reinstall the entire suite. There are two modes:

• Quick Repair — Fast, runs offline, replaces corrupted files from a local cache. Try this first.
• Online Repair — Slower, downloads fresh files from Microsoft’s servers, more thorough. Use this if Quick Repair does not help.

To run either: open Control Panel > Programs > Programs and Features, find Microsoft Office or Microsoft 365 in the list, right-click it and choose Change, then select your repair option.

Fix 8: Check for Windows and Office Updates

Microsoft regularly releases performance patches and bug fixes for both Windows and Office. Running an outdated version means you may be affected by known bugs that have already been resolved upstream.

1. Open Settings > Windows Update and install any pending updates.
2. In Outlook, go to File > Office Account > Update Options > Update Now to check for Office updates separately.
3. Restart your machine after updates complete.

If you are on a managed corporate device, your IT team may control update scheduling — check with them if you cannot apply updates manually.

Fix 9: Check Your Hardware

Software fixes will only take you so far if the underlying hardware is the bottleneck. Outlook is more demanding than it looks — the minimum specs Microsoft lists are enough to run it, but not to run it smoothly alongside other applications.

• RAM: Outlook works best with at least 8 GB of RAM. On a machine with 4 GB, any other open application will force Windows to swap memory to disk, making everything sluggish.
• Storage: If your machine still has a traditional spinning hard drive (HDD), upgrading to a solid-state drive (SSD) is the single biggest performance improvement you can make. Outlook reads and writes its data files constantly — an SSD makes this vastly faster.
• CPU: Outlook is not particularly CPU-hungry under normal use, but very old processors (pre-2015) will struggle with large mailboxes and multiple add-ins running simultaneously.

If you have recently set up Outlook on a new machine, our guide on how to set up Outlook on a new PC covers the account configuration steps that can also affect performance if done incorrectly.

Frequently Asked Questions

Why is Outlook slow only when I search?

Slow search is almost always caused by a damaged or incomplete Windows Search index. Work through Fix 4 above to rebuild it. If search remains broken after rebuilding, check that Outlook is listed as an indexed location under Indexing Options and that the Windows Search service is running in Task Manager.

Does a large number of emails actually slow Outlook down?

Yes, but it depends on whether those emails are cached locally. If your sync window is set to All (meaning every email is stored in your local OST file), a mailbox with tens of thousands of messages will create a very large file that Outlook has to manage on every sync. Reducing the sync window as described in Fix 3 is the most effective remedy.

Will disabling add-ins affect my Outlook features?

Only if the add-in provides a feature you actively use. Many add-ins are installed by third-party software without your knowledge and provide little or no day-to-day value. Disabling them is reversible — you can re-enable them at any time via the same Add-ins menu. Start by disabling one at a time and checking whether anything breaks before removing them all at once. You may also want to check if your Outlook autocomplete stops working after any changes, as some add-ins interact with that feature.

How big should my PST or OST file be?

Microsoft’s hard limit for PST files is 50 GB, but performance starts to degrade well before that. As a general rule, try to keep your active data file under 10 GB. If your OST file is significantly larger than that, reducing your sync window (Fix 3) and archiving old email (Fix 5) are the two most direct ways to bring it down. You can check your current data file size under File > Account Settings > Data Files.