Reminds me of Firefox. At the time, Mozilla/Netscape was very slow. Firefox, a small and very fast version of Mozilla, came out and took over.
React isn't slow (yet?), but it's not hitting any universal limits, there's always scope for leaner, smaller and faster software (yes it is, see benchmark).
benchmark: http://www.stefankrause.net/wp/?p=283
Edit: Added some reference benchmarks.