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.
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.