Again you can read into this what you will, but the speed index is an attempt to rate how quick your site appears to users, that isn’t the same as how big it is. Paul Irish of Google recently gave a really informative talk about performance
The item on mobile browsers and packets is interesting if a bit technical, but it really shows you how important it is to get the content to the users as quickly as possible.
Notes about the results:
Welsh sites: If you run against the root of all sites, then a lot of the Welsh sites score really high, but that is because most of them have a splash page asking the user to choose the English or Welsh version of the site. For fairness, I have tested Welsh sites against the English homepage not the splash page.
Open source everything: At Jumoo, we open source everything, so even though the python script is a bit of an abomination, you can go to github and see for yourself what we have done to reach these results.