When troubleshooting SharePoint (Online) performance issues, the SPIISLatency and SPRequestDuration headers are a very important part of the puzzle. Unfortunately, Microsoft removed those headers from SPO - honi soit qui mal y pense.
Despite a few reappearances for a short time – I missed those headers every time when dealing with MS Support in SharePoint Online Performance Cases (https://sharepointinterface.com/2017/07/07/the-five-minute-page-performance-troubleshooting-guide-for-sharepoint-online/)
But now, it looks like the performance figures are back – but depending on the UI you chose, in different flavors:
Look for the perf-JSON-Object in the Source Code of the page:
Search for g_duration in the Source Code:
SPO Performance Troubleshooting just got easier again!