There are multiple factors that can cause O365 outlook slow performance. The reasons can be issues on a client’s computer (Hardware and Software), Network or Internet connection issues.

Solution 1: Check the Computer Hardware and Software Health:

1. In the search box on the taskbar, type Windows Security, and then select it from the results.
2. Select Device performance & health to view the Health report.

Note: If your device is managed by your organization, your administrator may not have granted you permission to view Device performance & health.

The Health report starts off by showing you the last time a Device Health scan was run. The time displayed should be basically the current time, as Windows Security tries to run a Device Health scan when you open the Device performance & health page.

Beyond the time of last scan you’ll see the status of the key areas that Device Health monitors:

• Storage capacity – Is your system running low on disk space?
• Apps and software – Is any of your software failing, or in need of an update?
• Battery life – Is anything putting an extra strain on your PC’s battery? You might not see this on on a desktop PC that is always plugged in.
• Windows Time service – Having your system set to the correct time is important for a lot of system processes. Windows Time service automatically synchronizes your system clock to an internet-based time service so your system time is always correct. If this service is off, or failing, Device performance & health will let you know so you can fix it.

Solution 2: Validate the Outlook Version

1. In Outlook, choose File.

1. Select Office Account.

If you don’t see Office Account, option, choose Help. You’ll see the product version on the right side of the page.

1. You’ll find your version and build number under Product Information. For example, yours may list Microsoft 365 Apps for enterprise, and you can see which apps it contains by their icons.
2. If you need to know whether you’re using the 32-bit version or the 64-bit version of Outlook, select About Outlook.

1. The top of the About Outlook box shows the version number and 32-bit or 64-bit distinction.

Note: If you find anything version below Outlook 2019. Please raise the request to Local IT team to upgrade version to O365.

Solution 3: Check the Items or Folders are archived properly as per the Retention Plan

How to check to see if auto-archiving is in operation:

1. In Outlook, open the File tab and click on Options (in the column on the left).
2. Click on Advanced, again in the column on the left.
   

1. Click on AutoArchive Settings… in the AutoArchive section to open its dialogue box:
   

The options here control the AutoArchive function, including its frequency, how old messages should be before they are archived, and whether items should be moved to another folder:

Solution 4: Change Exchange Cache Mode to 3 months

1. Click File > Account Settings > Account Settings.

1. Click the Exchange or Microsoft 365, and then click Change.

1. Under Offline Settings, check Use Cached Exchange Mode.

(If you’re a Microsoft 365 subscriber with semi-annual updates, under Offline Settings, check Use Cached Exchange Mode to download email to an Outlook data file.

4. Exit, and then restart Outlook.