Alternative for iMacros "Stopwatch" command

The STOPWATCH command of imacros measures the time in seconds between two STOPWATCH commands with the same identifier.

From the wiki:
This command can be used for website response time measurement. Using different IDs in one macro different processes can be timed separately from each other. By default the measured data is stored in the download directory of iMacros. You can change the file location with the !FILESTOPWATCH variable. The last measured response time value can be referenced in macro via the !STOPWATCHTIME variable.

How to re-create feature this with the open-source tool? It was great to profile website performance.

PS: I already checked the imacros page.

You can use the !runtime built-in variable for performance profiling. It gives you the current macro runtime in seconds:

See also the Demo-Download macro measuring file download times: