I do a fair bit of work speaking on and helping others troubleshoot SharePoint Online (SPO) performance issues, and in this tool's current release (v2.0.1), the Page Diagnostics Extension addresses many of the gaps that were present in v1.x of the extension. The tool is no longer limited to classic mode pages (it handles modern pages just fine), and this version provides prominent links to additional resources which help to educate and explain many of the factors influencing SPO page performance. Scott Stewart and his team at Microsoft have really done all of us a great service by putting this together and making it freely available. In the overwhelming majority of cases, this tool provides enough data and guidance that I no longer feel the need to drop back to other browser tools, Fiddler, or Wireshark. Thank you!

As of this writing, this extension does not work with Modern SharePoint pages, only Classic pages. It notifies you if you run it on a Modern page that it may produce unreliable results, so be aware.

It's good tools, but is always active and flood console box with useless log ... please add disable button or delete the useless log : "Copying network performance info... contentScript.js:52 CustomEvent {isTrusted: false, detail: {…}, type: "SP-YSLOW-WINDOW-PERF", target: document, currentTarget: document, …}"

This app really helped me troubleshoot a performance issue with my SharePoint Online site. The only problem is though for what ever reason it broke some other sites when it was enabled. I'm not sure why it does this but luckily Chrome makes it easy to enable when I need to troubleshoot and disable when I'm not using it