So you are fully up-t0-date with Outlook however you are having issues with performance

1. The first thing Microsoft 365 support will do it recreate your profile, and also recreate your OST , so make sure you have done this
   
   
2. Next they will get you to run this :  Microsoft Support and Recovery Assistant for Office 365 (also known as ‘SaRA’). If will give you a list of problems you can work towards resolving
   
   
3. Make sure you upgrade to 64bit Version of Office

Outlook display issues only affect 32-bit versions of Office. Normally, 32-bit applications can only access 2 GB of memory that all 32-bit apps share. Outlook appears to handle this limit poorly, making it unusable once available memory dips below what Outlook requires to run smoothly.

To see how much memory is available for Outlook, try the VMMap Sysinternals tool, which shows the private bytes or memory distributed for a process that cannot be shared. VMMap shows the bit version of Outlook, as well. For 32-bit Outlook, a number at 250,000 K or less is bad.

Use the following registry setting to prevent add-ins from using the extra memory available from LAA mode. Change the xx to 15 for Outlook 2013 or 16 for Outlook 2016:
HKEY_CURRENT_USER\Software\Microsoft\Office\xx.0\Outlook\ReserveHighAddressRange DWORD Value ‘1’