A HAR file contains information that a web browser generates about how the browser loads a page's elements. It can be useful to help determine if specific elements may be slowing the loading process.
Important Note:
Instructions may vary by browser. The instructions and images below were captured in Google Chrome.
Instructions
Navigate to the site home page (or page in question)
Press F12 on your keyboard to open browser tools
Select Network from the list of tabs
Press F5 on your keyboard to refresh the page
Wait for the page to load fully; some scripts may continue to load once the page has finished
Check the Preserve log toggle
Right-click in the data area and select Save all as HAR with content
Save the file to your computer
Resources
Browser-specific instruction and .HAR file analyses are available via the Google Admin Toolbox's HAR Analyzer.