Outlook Is Slow: Why It Happens and How to Fix It

If Outlook is slow—taking forever to start, lagging when you switch folders, freezing during search, or showing "Not Responding"—work through the fixes below in order. They're written for Outlook on Windows (classic Outlook and the new Outlook for Windows) and focus on the most common "local" causes of Outlook performance issues: add-ins, oversized caches/data files, Windows Search indexing, and a damaged Outlook data file or profile.

Published on•April 22, 2026

Last updated on•April 23, 2026

13 min read

Michael Bodekaer Author

Founder, Board Member

Abraham Ranardo Sumarsono Reviewer

Full Stack Engineer

Outlook Is Slow: Why It Happens and How to Fix It

If Outlook is slow—taking forever to start, lagging when you switch folders, freezing during search, or showing “Not Responding”—work through the fixes below in order. They’re written for Outlook on Windows (classic Outlook and the new Outlook for Windows) and focus on the most common “local” causes of Outlook performance issues: add-ins, oversized caches/data files, Windows Search indexing, and a damaged Outlook data file or profile.

What’s new: One quick note: Microsoft has removed the Support and Recovery Assistant (SaRA) command-line utility from in-support Windows updates released on and after March 10, 2026, and recommends the Get Help command-line tool instead. ¹ If an older guide tells you to “run SaRA,” you may not have it—so the steps below stick to built-in Outlook and Windows tools.

TL;DR

Fastest fixes to try first

Update + restart: update Outlook/Office and restart Windows (Step 3).
Make sure Outlook actually closed: end any lingering Outlook process in Task Manager (Step 4).
Classic Outlook: run Safe Mode to check whether add-ins are the cause (Steps 5–6).
Search problems: rebuild the Windows Search index for Outlook (Step 8).
Freezing / “Loading Profile”: repair PST or rebuild the OST cache, then create a new profile if needed (Steps 10–11).

Step-by-step: Fix Outlook slow performance (do these in order)

Fix Outlook slow performance (step-by-step)

Identify your Outlook version and the exact slow moment
Open Outlook and check the top-left area: if you see File (and File > Options ), you’re in classic Outlook . If you don’t see the File tab and most settings live under a gear icon , you’re likely in the new Outlook for Windows . Write one sentence that describes the slowdown: “Outlook is slow when I start it / switch folders / search / open attachments / type.”

Check: You can say “I’m on (classic/new) Outlook, and it’s slow when I ____.”
Do a 2-minute “server vs PC” test
If you use Microsoft 365 or Outlook.com, open Outlook on the web in your browser and do the same slow action (open the Inbox, search, open a message). If the web version is also slow, your issue may be network/service-related—pause here and try again later on a different network. If the web version is fast, keep going: your local Outlook setup is the likely bottleneck.

Check: You know whether the slowness is “everywhere” or “only on this PC.”
Update Outlook, then restart Windows (don’t skip the restart)
In classic Outlook : go to File > Office Account (or Account ) > Update Options > Update Now . In new Outlook : open Microsoft Store > Library > Get updates . Then restart: Start > Power > Restart .

Check: After reboot, Outlook stays responsive for 5 minutes of normal use.
Fully close Outlook (make sure it’s not stuck in the background)
Close Outlook. Press Ctrl + Shift + Esc to open Task Manager . In the Processes tab, select Microsoft Outlook and click End task if it’s still running. Reopen Outlook and re-test the slow action.

Check: Outlook opens one time “cleanly” (no lingering OUTLOOK.EXE).
Run classic Outlook in Safe Mode to confirm (or rule out) add-ins
Classic Outlook only: close Outlook, press Win + R , type outlook.exe /safe , and press Enter . Use Outlook for 2 minutes (open a few emails, switch folders, search). If Safe Mode feels fast, an add-in is a very common cause of Outlook performance issues. ²

Check: You can say “Safe Mode is fast” or “Safe Mode is still slow.”
Disable add-ins the “one-by-one” way (so you actually find the culprit)
Open Outlook normally. Go to File > Options > Add-ins . At the bottom, set Manage to COM Add-ins and click Go . Clear every add-in you don’t need today (start with non-Microsoft add-ins), click OK , then restart Outlook. If Outlook becomes fast, re-enable add-ins one at a time (restart after each) until the slowdown returns—then leave that add-in disabled.

Check: You’ve identified one add-in that triggers the slowdown, or you’ve ruled add-ins out.
Reduce the amount of mailbox data Outlook loads (big win for large mailboxes)
Classic Outlook + Microsoft 365/Exchange: go to File > Account Settings > Account Settings . Select your main account > Change and move Mail to keep offline to a shorter window (for example, 3–6 months), then restart Outlook. If you have shared mailboxes you rarely use, remove them from the profile (or ask IT to do it) and restart again.

Check: Startup and folder switching are noticeably faster after the restart.
If Search is slow or missing emails: rebuild the search index
In classic Outlook: File > Options > Search > Indexing Options , and confirm Microsoft Outlook is included. Then open Control Panel > Indexing Options > Advanced > Rebuild . Keep Outlook open and leave your PC on until indexing finishes (search results can be incomplete while it rebuilds). ³

Check: Searching for a recent sender or subject returns results consistently.
Clean up your mailbox, then compact your PST/OST (this can remove “dead weight”)
In classic Outlook, use File > Info > Tools > Mailbox Cleanup and empty Deleted Items . Next, find oversized messages by clicking in the Search box and typing messagesize:>5 MB . Finally, run Compact Now : File > Account Settings > Account Settings > Data Files > select your data file > Settings > Compact Now . Outlook only compacts when there’s sufficient unused space in the file, so you’ll get the best results after a real cleanup. ⁴

Check: Outlook stops stalling when you switch folders or open large threads.
Outlook recovery step: repair PST corruption (or rebuild the OST cache )
If you use a PST (common with POP accounts or local archives), run the Inbox Repair Tool: close Outlook, open SCANPST.EXE , select your PST, click Start , then Repair . If you use Exchange , the OST is a local cache: you can close Outlook, delete the OST, and Outlook will recreate it the next time you open the app. Back up your data file before repairs; you may need to run the tool multiple times for stubborn PST errors. ⁵

Check: Outlook opens without freezing, and folders load without “Not Responding.”
Create a fresh Outlook profile (simple profile recovery)
Close Outlook. Open Control Panel and search for Mail (it may appear as Mail (Microsoft Outlook) ). Select Show Profiles > Add , name the new profile, and add your email account. When you’re done, set Outlook to use the new profile (or to prompt you to choose a profile at startup), then open Outlook again. ⁶

Check: The new profile is fast before you add extra mailboxes or add-ins.
Repair Office (and keep working with a backup plan)
If Outlook is still slow, run an Office repair: go to Settings > Apps > Installed apps > select Microsoft 365 (or Office) > Modify and run Quick Repair . If that doesn’t help, run Online Repair (it takes longer, but replaces more components). ⁷ Need to keep working today? Use Outlook on the web for a few hours, or switch your day-to-day inbox to an email client like Mailbird while you continue troubleshooting.

Check: After repair + reboot, Outlook stays responsive during a normal work session.

Tip: After each step, test the same 2–3 actions (startup, switch folders, search, open attachment). That makes it obvious which change actually fixed the slowdown.

Why Outlook gets slow (and why these fixes help)

Slow Outlook usually comes down to one of four things: an add-in hooking into startup or message processing, too much mailbox data being cached locally, a broken Windows Search index, or a corrupted data file/profile. The steps above isolate each cause and either remove the trigger (add-ins), shrink what Outlook has to handle (cache + compaction), or rebuild the broken parts (index, OST/PST, profile).

Outlook slow troubleshooting (symptoms and fixes)

Symptom	Likely cause	Do this
Outlook is fast in Safe Mode, slow in normal mode	An add-in is slowing Outlook down	Do Steps 6 (disable add-ins) and re-enable one-by-one.
Search returns missing/partial results	Indexing is incomplete or broken	Do Step 8 (rebuild the search index) and wait for it to finish.
Outlook hangs when switching folders	Huge mailbox cache, shared mailbox caching, or bloated OST	Do Step 7 (reduce offline mail window / remove shared mailboxes), then Step 10 (rebuild OST).
Outlook freezes when opening attachments	Antivirus scanning, add-in integration, or Preview Pane overhead	Disable add-ins (Step 6). Temporarily turn off the Reading/Preview Pane and test again.
PST/OST size won’t shrink even after deleting lots of mail	Deleted items weren’t emptied, or the data file still has too little reclaimable space	Empty Deleted Items, then run Compact Now again (Step 9). Leave Outlook open and idle for a bit.
Outlook takes forever at “Loading Profile”	Corrupted profile or stuck local cache	Create a new profile (Step 11). If needed, rebuild the OST first (Step 10).
New Outlook feels slow but classic Outlook is fine	Network/VPN/proxy issue, or the app needs a repair/reset	Try a different network, then repair/reset the Outlook app (see Variations below).
Nothing helped	Deeper Office install corruption or account-side issue	Run Online Repair (Step 12). If web Outlook is also slow, contact your email provider/IT.

Variations by Outlook version and setup

If you’re on the new Outlook for Windows: open Settings > Apps > Installed apps > Outlook > Advanced options and try Repair first, then Reset (you’ll sign in again).
If you’re on a corporate VPN or proxy: disconnect from VPN, restart Outlook, and re-test. If it’s suddenly fast, the fix is usually IT-side (routing, inspection, or allowlisting).
If your mailbox is massive but you can’t delete mail: shorten “Mail to keep offline” (Step 7) and rely on web search for older mail until Outlook behaves.
If you use PST archives for years of email: split “active” mail and “archive” into separate PSTs, keep the active PST small, and compact regularly (Step 9).

Prevent Outlook from getting slow again

Make-ahead (5 minutes a week)

Empty Deleted Items and clear out the Outbox if anything is stuck.
Uninstall add-ins you don’t use anymore (not just “disable”).
Keep attachments out of email threads when possible (share links instead).

Maintenance (15 minutes a month)

Run Mailbox Cleanup and then Compact Now for your main data file (Step 9).
Audit shared mailboxes: remove any you don’t need opening automatically (Step 7).

Scaling for teams (help desk-friendly)

Standardize a “known-good” add-in list and remove everything else by default.
Create an internal runbook that mirrors Steps 5–12 so every technician tests the same way.
Offer a fallback client plan (Outlook on the web or a desktop client like Mailbird) so work doesn’t stop during troubleshooting.

Frequently Asked Questions

Why did Outlook become slow “all of a sudden”?

Most sudden slowdowns come from a new/updated add-in, a mailbox that crossed a size threshold, a search index that got stuck, or a Windows/Office update that needs a restart to finish cleanly. Start with Steps 3–6.

How do I know if an add-in is the problem?

If classic Outlook is fast in Safe Mode but slow in normal mode, disable add-ins and re-enable them one at a time until you find the one that brings the slowdown back (Steps 5–6). ²

Will rebuilding the Outlook search index delete my email?

No—rebuilding the index rebuilds the search catalog. Your email stays in your mailbox/data files. Searches may be incomplete until indexing finishes.

Is it safe to delete an .ost file?

If your mail is stored on the server (for example, Microsoft 365/Exchange) and the OST is just a local cache, Outlook can rebuild it. Don’t do this if you’re unsure whether you have local-only data. ⁵

How do I repair a corrupted .pst file?

Use the Inbox Repair Tool (SCANPST.EXE) with Outlook closed (Step 10). If it still finds errors, run it again, then re-open Outlook and look for recovered folders. ⁵

Why doesn’t my PST/OST shrink after I delete a lot of email?

Deleting mail creates “free space” inside the data file, but the file size may not drop right away. Empty Deleted Items and run Compact Now (Step 9). ⁴

What if my company blocks these settings?

Do the non-invasive steps first (web vs desktop test, updates, restart, Safe Mode test). If settings are locked (cache window, shared mailbox behavior, or add-ins), send your findings to IT and ask them to apply the change through policy.

I need email working today. What’s the fastest workaround?

Use Outlook on the web for the affected account, or use another desktop email client (like Mailbird) temporarily so you can keep sending/receiving while you run Steps 6–12.

Quick checklist (screenshot this)

Identify classic vs new Outlook, and write the exact slow action (startup/search/attachments/etc.).
Test Outlook on the web to separate “server/network” from “PC/app.”
Update Outlook/Office and restart Windows.
End any stuck OUTLOOK.EXE process in Task Manager, then re-test.
Classic Outlook: run outlook.exe /safe and compare speed.
Disable add-ins and re-enable one-by-one to find the culprit.
Reduce offline mail cache window (Exchange/M365) and remove unused shared mailboxes.
Rebuild the search index if Search is slow or incomplete.
Mailbox Cleanup + Compact Now for PST/OST.
Repair PST with SCANPST (or rebuild OST cache for Exchange/M365).
Create a fresh Outlook profile.
Run Office Quick Repair, then Online Repair if needed.
New Outlook: repair/reset the app in Windows Settings (see Variations).