The Outlook Mail on your computer may be slow and sometimes fail to load. Try any of these practical solutions to fix Outlook Mail that is running slowly.
Click on File, and select Info.
Next to Mailbox Settings, click on Tools.
Choose “Clean Up Old Items.”
Select the folders you wish to archive.
Select a date and archive location.
Click on “OK.”
Open the folder that is opening slowly.
Find the “By” option or sorting menu and click on it.
Select the type of heading you want to organize your contacts or emails.
For contacts: sort by Name.
For emails: sort by Date Received or Subject.
Create a PST folder in Outlook.
Click on File and go to Account Settings.
Select Account Settings, then navigate to Data Files.
Choose a location on your computer.
Name the file LocalBackup.pst and click OK.
Return to your Inbox.
Select the emails you want to move. Use Ctrl + A to select all emails in a folder.
Drag and drop the emails into the new PST folder.
Once moved, the messages are saved only on your computer, not on the mail server.
Press the Windows + R keys on your keyboard.
The Run dialog box will open. In the given field, type outlook.exe /safe and click on OK.
The Outlook Mail will open in Safe Mode.
Check whether it runs faster or not.
Source:- Fix Outlook Running Slow