Not sure, but the time cost is the latency of loading an almost-empty index.html, loading and parsing your app, _then_ it starts running and hopefully fast doing some API requests and then it's off and good to go. Subsequent JSON-API-requests-and-then-rerenders should be pretty fast indeed, but those aren't the point :)

You want to server-side render React apps so that while all that happens, the app already works.