Slow can certainly be a negative signal, I agree (though if the content is mostly just HTML and CSS without much media then speed only means so much).

But depending on the topic, "stale" may not mean much about quality at all. I've had a FAQ site about Tolkien's books online for at least 15 years now, and as it turns out he hasn't been publishing much lately (being dead and all). So apart from minor updates every few years when his son or other researchers publish new information from his drafts and notes, the site has been largely static for a decade or more. It's very good at its intended purpose, and it's hard to imagine what I could do to provide a steady stream of new content to indicate "freshness" without fundamentally changing the nature of the site.

I imagine that the same is true of a lot of sites out there, on a lot of topics that are far removed from the "breaking news" world.