You need to confirm whether the performance issues experienced by the HR department site collection are due to the large image.
What should you do?
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