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.
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.