Outlook Running Slow: How to Fix It

A slow Outlook is one of those problems that creeps up gradually — emails take a second longer to load, search starts returning results slowly, and eventually opening the app itself becomes a thirty-second wait. The good news is that every common cause has a direct fix, and most of them take under five minutes to apply.

Start Here: Check Your OST or PST File Size

Classic Outlook stores your mailbox locally in an OST file (for Exchange/Microsoft 365 accounts) or a PST file (for POP/IMAP or archived mail). Over time these files grow large and become fragmented, which makes everything — opening Outlook, switching folders, loading emails — noticeably slower.

To find your data file location, go to File > Account Settings > Account Settings > Data Files tab. Note the path and check the file size in File Explorer. If your PST is over 10 GB, compact it:

1. Go to File > Account Settings > Account Settings > Data Files.
2. Select the data file and click Settings.
3. Click Compact Now and let it run.

If your mailbox is overloaded with old emails, see our guide on fixing a full Outlook mailbox for steps to archive and clean up properly.

Reduce How Much Mail Outlook Caches Locally

By default, Outlook for Microsoft 365 caches your entire mailbox locally. Reducing the sync window makes a significant difference.

1. Go to File > Account Settings > Account Settings.
2. Double-click your Exchange or Microsoft 365 account.
3. Under Offline Settings, drag the Mail to keep offline slider down — try 3 months or 1 month.
4. Click Next then Done.

Disable Slow or Unnecessary Add-ins

Add-ins are the single most common cause of Outlook slowdown. Antivirus integrations, CRM plugins, Teams Meeting add-ins, and Grammarly are all regular culprits. Outlook will sometimes warn you an add-in is slowing startup — check for that notification at the top of the app.

1. Go to File > Options > Add-ins.
2. Set the Manage dropdown to COM Add-ins and click Go.
3. Uncheck any add-ins you don’t actively use.
4. Restart Outlook and check whether it’s faster.

To confirm an add-in is to blame, open Outlook in Safe Mode (Win + R, type outlook /safe). If Outlook is fast in Safe Mode, an add-in is definitely causing the slowdown.

Turn Off Hardware Acceleration

On certain graphics card drivers or remote desktop sessions, hardware acceleration causes sluggish scrolling, flickering, or brief freezes when switching between emails.

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

Fix Slow Outlook Search

If search crawls or returns incomplete results, the Windows Search index for your Outlook data file is broken or out of date.

1. Go to File > Options > Search.
2. Click Indexing Options.
3. Click Advanced, then under Troubleshooting, click Rebuild.
4. Confirm and wait — rebuilding can take 20–40 minutes in the background.

For more search-specific fixes, see our guide on Outlook search not working.

Clear the Autocomplete Cache

The autocomplete cache — the list of email addresses that appears when you start typing in the To field — grows over time and can contribute to sluggishness.

1. Go to File > Options > Mail.
2. Scroll to the Send Messages section.
3. Click Empty Auto-Complete List and confirm.

Check Your Antivirus Email Scanning Settings

Some antivirus products scan every email as it arrives through Outlook’s own integration. This real-time scanning adds latency to every email you receive and open. If you’re running a product with an Outlook plugin (Norton, McAfee, ESET), try disabling the Outlook-specific integration — you’ll still be protected by network-level and on-access scanning without the per-email overhead.

New Outlook: Performance Tips

The new Outlook for Windows is a web-based application running on Edge WebView2. Its performance profile is different from classic Outlook.

• Slow to open: Try signing out and back into new Outlook to clear cached session data.
• Slow search: New Outlook uses server-side search — if search is slow, it’s usually a Microsoft 365 service issue. Check the Microsoft 365 service health dashboard.
• Add-ins: Go to View > Add-ins to manage web-based add-ins. Removing unused ones helps with load time.
• General sluggishness: New Outlook is more demanding on RAM than classic Outlook. If your machine has 8 GB or less, consider staying on classic Outlook.

When to Consider a Full Repair

If you’ve worked through all of the above and Outlook is still slow, run an Office Online Repair. Go to Control Panel > Programs and Features, right-click Microsoft 365 or Office, choose Change, and select Online Repair. Allow 15–20 minutes and a restart. Creating a new Outlook profile is also worth trying before a full reinstall — a corrupted profile causes erratic slowness that no other fix will solve.

Use Task Manager to Diagnose the Slowness

Before trying fixes at random, it helps to know whether Outlook itself is the bottleneck or whether something else on your machine is consuming the resources Outlook needs.

Open Task Manager with Ctrl + Shift + Esc and click the Processes tab. Sort by CPU or Memory to see which applications are using the most resources while Outlook is running. If Outlook (OUTLOOK.EXE) is consistently above 20–30% CPU while idle, the most likely causes are a runaway add-in, real-time antivirus scanning of the OST file, or a corrupted OST that is being re-indexed continuously.

If another process — such as MsMpEng.exe (Windows Defender) or a third-party antivirus — is peaking whenever you open Outlook, exclude your Outlook data folder from real-time scanning. The default location is C:Users[username]AppDataLocalMicrosoftOutlook. Adding this folder to your antivirus exclusions can make an immediate and dramatic difference to Outlook’s responsiveness without reducing your overall protection.

Reduce the Size of Your OST File

Over time, the local Outlook data file (OST) grows as emails, attachments, and calendar items accumulate. Very large OST files — particularly those over 10 GB — can slow down every Outlook operation including opening, searching, and switching folders. To compact the file:

1. Close Outlook.
2. Open Control Panel > Mail > Email Accounts > Data Files.
3. Select your data file and click Settings.
4. Click Compact Now and allow it to finish before reopening Outlook.

If your OST is very large and you use Microsoft 365, also consider enabling Online Archive to move older items server-side, or reducing the amount of mail cached locally via File > Account Settings > Change > adjusting the “Mail to keep offline” slider.

For a full index of every Outlook guide and troubleshooting fix on Serverman, see the Microsoft Outlook complete guide and troubleshooting hub.