How to measure the Network Round Trip Time to Office 365 (older article but still very usefull)
https://blogs.technet.microsoft.com/onthewire/2014/05/08/how-to-measure-the-network-round-trip-time-to-office-365/
performance
Outlook performance issues when there are too many items or folders in a Cached mode .ost or .pst file folder
https://support.microsoft.com/en-us/kb/2768656
Office 365 performance – image renditions causing slow page loads in SharePoint Online
http://www.sharepointnutsandbolts.com/2016/03/office-365-sharepoint-performance-image-renditions.html
SharePoint Online performances
https://yos365.wordpress.com/2016/01/05/espc15-part-3-sharepoint-online-performances/
Best Practices for Design and Performance in SharePoint Online
https://myignite.microsoft.com/#/videos/a9c3a303-3695-e411-b87f-00155d5066d7
Tune and optimize performance of your Office 365 connection
http://blogs.office.com/2014/10/29/tune-optimize-performance-office-365-connection/