Render is a different thing in both places. Server Renders is ... formatting a sequence of bytes. Hopefully your templating engine is optimized and it is a low cost operation. Rendering on the client is obviously more involved as it requires actual rendering on the screen using the graphics drivers.

On the other side of the scale, React has overheads like a virtual dom.

It is a complicated question as to what is most performant, who bares that cost, and so on.