Do you mount old tires on your new car?
Are you buying new shoes and moving over the soles of your old ones?
Then you are also likely to run Office 365 with your old, traditional network settings
If you experience that one or more Office 365 services are slow, you may want to analyze your settings and make some corrections.
I recommend reading Tony Redmond’s article “Network Principles for Office 365 Connectivity” where he is describing a common problem. Perhaps you can easily do something about it