There may be some fast components in the Reddit web app, but I think there must be a valid reason why so many users are switching to the old Reddit (not just because of the way it looks, but due to performance complaints). I just went in and even scrolling through Reddit comments is laggy. Switching to old mode is like day and night.
I built a SPA recently and learned pretty quickly to replace `on('scroll', handler)` with a set-flag-and-sweep-at-slower-interval solution to bring down CPU usage.
While something like React alone isn't slow, building a SPA means you're taking on a lot more responsibility, and there's a lot more things that can go wrong.
Even something like "when user clicks an item, make loader spin until server response, and then render the detail page with the data" can feel much, much slower even though it's not from a clock-time standpoint.
Aside: Using a slow device as your main testing device is a life-changer if you want to catch this stuff early. It's hard to ignore this stuff when you have to feel the pain yourself on every refresh. Good reason to always keep your old phone around.
I just had to come back to this comment after testing in a couple of browsers, focusing on Safari and Chrome.
Going to https://www.reddit.com/r/whatisthisthing/comments/9oz5sa/wha... and scrolling is OK in Safari but actually pretty laggy compared to old mode. It's definitely not how a modern web site should perform. Testing with Chrome, it was fast in the beginning, but if you scroll up and down for about 15-20 seconds, CPU load goes up and my laptops fan start spinning.
So no, I do not believe reddit's new design is "performant" at all.
One has to wonder - if Reddit's engineers can't get React right (I'm sure they hire smart people), is it really not a problem with React in the end?
To me the Reddit web app is fast, the show part I think is the server side.