Explanation
Scenario: Users in the HR department report performance issues affecting their site collection. You suspect that the issues are due to large images on the home page.
You can diagnose common issues with your SharePoint Online site using Internet Explorer developer tools.
There are three different ways that you can identify that a page on a SharePoint Online site has a performance problem with the customizations.
* The F12 tool bar network monitor
* Comparison to a non-customized baseline
* SharePoint Online response header metrics
References:
https://docs.microsoft.com/en-us/office365/enterprise/diagnosing-performance-issues-with-sharepoint-online