The issue seems to be down to your network connection. As Outlook is sending or receiving emails and making other service calls you experience a slowdown. Onsit

The issue seems to be down to your network connection. As Outlook is sending or receiving emails and making other service calls you experience a slowdown. Onsite our connection is very slow and everyone experiences this problem. Additional bandwidth was not considered for the O365 and if you monitor your service calls with fiddler for example you'll see constant traffic to the O365 servers.