Classic Outlook in Microsoft 365 can run very slowly even on a new Windows 11 laptop. Many users notice that the Send/Receive bar moves slowly, emails stay in the Outbox, or inboxes take a long time to sync. This usually happens when Outlook uses IMAP accounts like Gmail, antivirus software scans email traffic, or the Outlook data file becomes too large.
This guide explains how to fix slow Classic Outlook step by step using safe methods that work on Microsoft 365 personal accounts, home office setups, and Windows 11 PCs.
Why Classic Outlook Runs Slow
Outlook slows down when the sync engine cannot process mailbox data fast enough. This problem usually comes from software settings, not internet speed.
Common causes include:
- Antivirus scanning Outlook emails
- Corrupted OST or PST file
- Too many add-ins
- Large mailbox sync
- Old Outlook profile
- Hardware acceleration issues
- IMAP accounts like Gmail
Even with fast internet, Outlook can still run slowly because most delays happen inside the program.
Fix 1 — Disable Antivirus Email Scanning
Antivirus software often checks every email that Outlook sends or receives. This makes the Send/Receive bar move very slowly.
Open your antivirus settings and look for email protection options.
Disable features such as:
- Email scanning
- Outlook plugin
- SSL scanning
- Mail protection
Restart Outlook after changing the settings.
If Outlook becomes faster, the antivirus caused the delay.
Do not uninstall antivirus. Only disable email scanning.
Fix 2 — Create a New Outlook Profile
Old Outlook profiles become slow over time, especially when you use multiple accounts.
Follow these steps:
- Close Outlook
- Open Control Panel
- Search for Mail
- Click Mail → Show Profiles
- Click Add
- Create a new profile
- Add your email accounts again
- Select Always use this profile
- Choose the new profile
- Start Outlook
A new profile often fixes slow sync, Outbox delays, and Send/Receive problems.
Fix 3 — Disable Unnecessary Add-ins
Add-ins run inside Outlook and can slow down sending, syncing, and startup.
Disable them using these steps:
- Open Outlook
- Click File → Options → Add-ins
- At the bottom select COM Add-ins → Go
- Uncheck everything except Microsoft add-ins
- Click OK
- Restart Outlook
If Outlook becomes faster, an add-in caused the problem.
Fix 4 — Repair Outlook Data File (PST / OST)
A damaged data file can make Outlook freeze, lag, or sync slowly.
Repair the file using the built-in tool.
Steps:
- Close Outlook
- Open File Explorer
- Go to
C:\Program Files\Microsoft Office\root\Office16\- Find SCANPST.EXE
- Run the tool
- Select your file from
Documents → Outlook Files- Click Repair
- Start Outlook again
This fix works when Send/Receive gets stuck or emails stay in the Outbox.
Fix 5 — Reduce Mailbox Sync Size (Important for Gmail)
IMAP accounts like Gmail sync every folder. Large mailboxes make Classic Outlook slow.
Reduce the sync load using these steps:
- Open Outlook
- Click File → Account Settings → Account Settings
- Double-click your email account
- Click More Settings → Advanced
- Reduce sync size if available
- Archive or delete old emails
Large mailboxes always slow down Classic Outlook.
Fix 6 — Disable Hardware Acceleration
Graphics acceleration can cause lag in Outlook on Windows 11.
Follow these steps:
- Open Outlook
- Click File → Options → Advanced
- Scroll to Display
- Enable Disable hardware graphics acceleration
- Click OK
- Restart Outlook
This fix often improves performance on new laptops.
Fix 7 — Update Microsoft 365
Old Office builds can cause sync bugs and slow performance.
Update Office using these steps:
- Open Outlook
- Click File → Office Account
- Click Update Options → Update Now
- Install updates
- Restart Outlook
Always keep Microsoft 365 updated.
Fix 8 — Try the New Outlook (Optional)
Classic Outlook uses an older sync system. New Outlook uses a modern sync engine and works faster with IMAP accounts.
You can test it without removing Classic Outlook.
- Open Outlook
- Toggle New Outlook at the top right
- Test Send/Receive speed
If performance improves, the issue comes from the Classic Outlook engine.
When Outlook Runs Slow Even on a New PC
Slow Outlook does not always mean slow internet or weak hardware. Most problems come from antivirus scanning, profile corruption, or large mailbox sync.
If the Send/Receive bar moves slowly or emails stay in the Outbox, follow the fixes in this guide in order. One of them usually solves the problem.
FAQs:
Why is Outlook Send/Receive very slow?
Antivirus scanning, large mailbox sync, or a corrupted profile usually causes this.
Does Gmail make Outlook slow?
Yes. IMAP sync with large Gmail accounts can slow Classic Outlook.
Should I create a new Outlook profile?
Yes. A new profile often fixes slow sync and Outbox delays.
Can antivirus slow Outlook?
Yes. Email scanning can make Send/Receive very slow.
Is New Outlook faster than Classic Outlook?
Often yes, especially when using Gmail or IMAP accounts.
