You cannot really compare a React project with a non-React project if it is done by different people.
We use React since four months, and we develop faster compared to before, when we used only jQuery.
We use React since four months, and we develop faster compared to before, when we used only jQuery.
(Genuine questions, not making a point).
I think there are many reasons for the productivity boost. Thinking in components, more readable code, great debugging possibilities, understandable data flow, to name a few.
And, last but not least, it is lots of fun :-).