[_heimdall]

That depends heavily on the device and network being used for testing. In my experience 3-6 seconds isn't out of the question by any means for a decent sized react SPA loaded on an older device and mobile network.

That isn't necessarily specific to react, any client-side rendered framework will have issues. React tends to be noticeably worse in my comparisons though, and any client rendering is going to be worse than server rendering your HTML.