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.
I'm not talking about most performant. Neither HTMX nor React has a performance issue.
The issue is overhead and complexity for the developer, and maybe, server costs.
React comes out ahead in server costs, since doing work on the client is very cheap (provided there isn't a performance issue that turns users off, which there isn't).
And in terms of dev time, it depends, but you can't say "look, with HTMX you can avoid code" because you still do need to write code, but now it's in the server.
