Which react sites? Are you sure you are not a victim of confirmation bias? How often do you check what framework does a smooth and performance website uses?
Because demo sites are snappy on even low-end devices. It's not react itself that causes slowness, but all the other libraries/tracking/ads.
Which (correctly) doesn't even have react listed, only frameworks that (may) use react as a library.
So at least be specific what is problematic, is it a particular framework, a particular library, the whole idea behind react (v=f(s)) or what? Anything can be proven against something non-concrete.
Because demo sites are snappy on even low-end devices. It's not react itself that causes slowness, but all the other libraries/tracking/ads.