HAR File

The HTTP Archive format - HAR - is a JSON-formatted archive file format for logging of a web browser's interaction with a site. The common extension for these files is .har.

Typically data about timings is stored and can be used to generate a waterfall graph:

Time to fetch DNS information

Time for each object request

Time to connect to server

Time to transfer assets from the server to the browser of each object

HAR files can be generated by most browsers.