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?