React's virtual dom/diffing is certainly not "slow". It's not the tool that's the problem, but how it's used. There's also more to perceived slowness than how fast your library reconciles the DOM, e.g. I too often encounter pages that will "preload" all their assets behind stupid loading screens, ill-timed CSS animations, modals on top of modals, React Components that badly wrap jQuery plugins, leaked callbacks, …
Yea, I would say so... I guess from a developer standpoint it makes it easy to just have a "Post" component with various elements. I did this for a Reddit clone using Angular and ng-repeat and whatnot. I don't think it's performance beneficial though.
tl;dr Don't blame tools but their usage