[jvzr]

Yes, but Gmail doesn't really have to compete in Google Search, does it? So, in their eyes, it's OK for Gmail to score low because it has its own link in basically every navigation toolbar/header Google owns. This won't improve the speed at which Gmail loads, whereas all non-Google websites will have to improve their speed.

[koz_]

It's not that Gmail has a dominant position and so doesn't need to care about its rating - Gmail is an app not a page and it's less important for apps to load quickly than pages. Apps tend to be long lived and so a slow startup time is tolerable, whereas one is constantly opening new pages.

That is to say, I don't think e.g. Trello would really care if their site was "slow to load" either.