Yeah, but strictly as a solution to shifting layout, having metrics asap and actual text display later — when the glyph data is downloaded — is the most obvious

Yeah, but strictly as a solution to shifting layout, having metrics asap and actual text display later — when the glyph data is downloaded — is the most obvious solution. Rendering a similar system font (by metrics comparison, ideally) in its place is indeed a great improvement on this, but mostly for the render-as-soon-as-possible problem. I'd treat it as a separate problem simply because it can be on the user agent to decide to do it and how, and even without it, a bunch of web apps would start benefitting right away (those which have most of their actual text off screen but which might block rendering other parts of the page).