Add website size test or website carbon estimator
Since the introduction of the speed test in https://github.com/Seravo/seravo-plugin/commit/6ed54ae507e2cf082ce6f52179a1247b38e74596 customers have hade a nice UI to test the speed of their site.
There could also be something that would estimate the total download size of their site - a factor that directly correlates with speed (in particular over mobile connections) and also correlates with carbon emissions. Such a test should be done using the headless Chrome to capture the full page load.
Alternatively we could integrate a carbon emission estimator by integrating with websitecarbon.com:
https://www.websitecarbon.com/website/wp-palvelu-fi/

Both measures are mitigated by decreasing the total download size and would lead to similar behavior on website devs.
I think implementing this would be too much of an effort and still not very accurate. However, to emphasize this the Site info box could include a link to The Green Web Foundation check so site admins can discover that their site is green and listed there?