Microsoft has a tool available for measuring Office 365 performance.
The tool tests dns response time, latency to the datacenter where your SPO or Exchange are located, and bandwidth.
It then displays results in either green or red based on the performance you’re getting.
The article linked above has both the download link and an explanation of each value.
It’s a handy tool, in that you can give it to your users if they say SPO is slow, and collect the data back. It’s much easier than asking them to time a page load, plus the page load wouldn’t say why the load is slow, just that it is slow. This tool will help provide data that can explain the why.