It's not a React problem. React runs Facebook, Instagram, and mobile Twitter, for example. But it's possible to build inefficient React implementations. Reddit rebuilt their front end from the ground up. It is possible they have front end issues there. Some flame chart analysis on Reddit's current front end and old front end would reveal the source of the problems.
While I agree React isn't the issue here, you chose horrible examples. Loading Facebook on my computer (especially trying to open a chat window) brings my computer to a knee. And using twitter on mobile browser you quickly learn how the spinner looks.
I haven't had performance issues on Facebook in quite some time. And Twitter works like a charm on my Nexus. I'm wondering how much of your issues have to do with either round trip latency or network issues.
