Am a keen follower of Microsoft's SharePoint Blog and proud to provide this direct from the Microsoft Tech Community:

Today we’re pleased to announce availability of the next release of the Page Diagnostics Tool for SharePoint (Version 2.0.0) which now includes the ability to assess modern and classic SharePoint pages. 

The Page Diagnostics Tool for SharePoint is a Chromium Browser Extension (can be used in Chrome and  Microsoft Edge version 77 or later) designed to help you identify site pages in SharePoint Online that may not be delivering optimal performance.

The Page Diagnostics Tool works by comparing discovered characteristics of a site page in SharePoint Online to known best practices and provide guidance towards remediation of those issues to ensure you are delivering the best possible experience across these pages.  In addition to addressing performance of existing pages, the Page Diagnostics Tool can also be used to evaluate existing pages before they are published in your environment. As such it is recommended that you use the tool to evaluate new pages or changes to pages prior to publication in your production tenant or as part of your development life cycle in your development tenant.

The Page Diagnostics Tool is currently available as a Chrome Browser Extension that can be downloaded directly or installed through the Chrome web store.

For additional information on installing and using the Page Diagnostics Tool, refer to the getting started guide https://docs.microsoft.com/en-us/office365/enterprise/page-diagnostics-for-spo

The Page Diagnostic Tool also provides detailed information on a variety of characteristics useful for support and troubleshooting purposes to include the current CorrelationID which is helpful when engaging Microsoft support as well as page load time, and the current URL for the page being evaluated.  An additional preview feature has also been included to further assist when working with Microsoft support, “Enable exporting to HTTP Archive (HAR) – Preview”. This can be used to provide debugging data to the Microsoft support engineer.

For information on how to use this information, refer to support documentation for the Page Diagnostics Tool at https://docs.microsoft.com/en-us/office365/enterprise/page-diagnostics-for-spo

Rules will be added and adjusted with each release so please ensure you update the tool to the latest version. Each rule has an associated “Learn more” link which will take you to the supporting page providing more insights on the rule for the Attention Required (red) and Improvement Opportunities (yellow) sections so that you can review what actions to take.

Please use the additional resources section on the start page for the tool should you require additional information on how to use the tool or how to tune SharePoint online performance. 

You can also use the ellipse in the top right corner to get to these links.

You can also use the give feedback link to connect directly to UserVoice and log suggested functionality for the tool.

The About link will show you the current version as well as the third-party notice.

We look forward to your feedback as we continue to consider scenarios for future versions of the Page Diagnostics Tool for SharePoint and trust that the recommendations will guide you towards great performing pages.

The above is kindly provided by the Microsoft Tech Community!