Interesting. The ones not behind a login (Twitter,AirBnB,Reddit Mobile) are sites that feel laggy and clunky to me. With slow loading content, loading animations, laggy scrolling, too much stuff opening in new tabs, confusing interfaces.
I wonder if React causes/incentivises this or if it's design decisions by the coders/designers/managers.
In contrast, pages like Reddit Desktop, Google and Amazon work much better for me.
I don't contest that those sites feel laggy and clunky, whether by poor load time, poor UX/UI, or some larger dysfunction.
However, I would offer that the usage of React is not so much the cause of poor performance but rather a step taken to remedy scale lag. Large apps like the one you mention are big behemoths that take months to adopt new architectures and patterns, but those activities are required to ensure the scalability of the infrastructure and product development. During the time those apps are not fully transitioned to React (and Redux, and isomorphic rendering, and a fully-featured API, and webpack, and a litany of other tools), they might feel half-baked or sluggish, because you may have a page loaded with server-side template rendering, but half of your components on the page are upgraded to React, and take a second to initialize.
IMO, React is a tool that provides an appropriately rigorous rubric for component development and a structure for large-scale interplay of reusable/composable frontend components, which was required for it to have any viability for a large org like Facebook. A lot of web apps are on the long trek to avail that potential fully, but while they're in transition, the experience of those apps will be a little disjointed or degraded.
Big web properties like these tend to be developed by huge teams and tend to prioritize features over performance.
It's partly Conway's law and partly the result of having lots of users and features being easier to add than take away.
None of these things really have much to do with React. If your app was going to be slow anyway, React won't help you. However, like Angular, React makes it easier to break things down into components and work on them separately, so it's popular with the same big teams that make bulky and slow web pages, just like Angular was before it. Correlation, not causation.
Unlike Angular, React is also fantastic for small teams making lean pages that load blazing fast, which is why it's become so popular so quickly.
JS libraries (like React) are just tools developers have. They can make fast sites with them, and they can make slow sites with them. We use React because it makes it easier to optimise our site to make it load very fast.
Actually the new Twitter Mobile called Twitter Lite is super fast, did you try the latest version? It's a full progressive web app that works offline, has notifications and super fast scrolling. It was designed to work on low end Android device, give it a try: https://mobile.twitter.com
At first it gives me a white page with a little blue twitter logo. Then the twitter logo disappears and I see a white page for a second. Then it gives me a strange blue page with a giant twitter logo and two buttons glued to the bottom that tell me to log in.
I don't know why you're arbitrarily dividing the data into "behind a login" and, well, "not". I doesn't make any difference.
For the rest: opening in tabs is obviously a design choice. So are "confusing interfaces"–whatever that may mean, because if those reddit and twitter mobile sites are confusing you, good luck with amazon. React build a DOM. You can make a table-based layout with a spinning e-mail icon if that's what you want.
As for load times: mobile.reddit.com comes in at just under 1MB, but that's compressed to 400kb which is basically irrelevant once you expand the first image posts. JS is just about 200kb compressed.
And once it's loaded, only the JSON-encoded data goes over the wire: on the first click anywhere, the data transfer is amortised because the hundreds of <div class="asdasda"> don't have to be transferred.
I wonder if React causes/incentivises this or if it's design decisions by the coders/designers/managers.
In contrast, pages like Reddit Desktop, Google and Amazon work much better for me.