A bit of both. With React we now have a much nicer code structure, because it forces us to think in components. We have a JS file and an SCSS file for each component. Before we rendered the view on the server side with Python/Jinja2.

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 :-).