Are you sure it is not due to the fetching of content form the server rather than the UI synchronicity? I'm quite certain it is just latency for the download. The components might be fetching that data during their mounting if the architecture was set up that way, but the actual mounting shouldn't be the cause; React's UI is much faster than latency caused by that alone.
It would be better if the components showed a waiting indicator during the download, if the components are responsible for the download. Alternately, the page itself could show that loader before mounting its sub components. Actually, there are many "alternately"s because frankly this site's approach isn't ideal.