Their use model is specifically on mobile devices, and it is certainly true that many sites run absolutely abysmally on smartphones/tablets. While these devices are powerful, they're still weak compared to most of our desktop machines.

Flipboard spent an enormous amount of effort making a rendering to canvas platform, humorously to allow an "open" option aside from their app (which makes the complaint about it in the submission rather perplexing). I suspect they did a lot of analysis. And the DOM is notoriously slow, one of the reasons being that it is now a catch all/everything and the sink platform that has an enormous number of modifiers -- the flexibility that we hail is also what leads to engine slowdown. We need a new, simpler layout standard that simplifies all of the various sidepaths and diversions that got built into the standard.