In the case you experience slow sending and/or receiving of emails with Outlook 201x try one of the options below:
Enable Outlook with Cache
'Mail to keep online': set to 'All'
Start Outlook in Safe modes (outlook.exe /safe)
Or try with disable Outlook Cache
Create a new Outlook Profile
Does the computer has the latest patches/updates?
Test account on another computer with Outlook
Check your hard drive for bad sectors
Temporary disable Anti-Virus , restart Outlook and try again
Is domain complete installed / validated in Office 365 Admin center?
Other related (Microsoft) articles:
Microsoft Cloud / Microsoft Office 365 / Tips with Outlook connections issues
Outlook 2016 autodiscover takes a long time to complete