yes @yokunjon, it is almost all server side with a very minimal client. Obviously there needs to be a small hyper text document and a little JavaScript to connect the phoenix Channel to the client's browser. But no, despite being rendered on the server side this is not very resource intensive. Elixir is an extremely efficient and preformant language because of its concurrency model (the actor model) and so when you start working with these Phoenix Channels you are able to harness that distribution and take full advantage of the system. When they announced LiveView a few months ago, they demoed it with an animation that ran over 60 fps with very minimal resource usage which was interesting.
I know the syntax can be a bit of a nightmare at first; all of this pattern matching can be confusing and writing callback functions as first class functions can seem a bit strange at first as well. That being said, you could easily learn Elixir in a few weeks given some time. I find that it is one of the more accessible functional languages out there and its actually a fairly accessible language in general. You just have to get over the initial abstraction hump for the Actor Model and once you've done that its pretty intuitive. I've actually got an Intro to Elixir series that I am working on that you are welcome to follow along with.
Anyhow, thanks for moderating my contribution.
RE: Phoenix Framework - Building a Chat Room with Phoenix Live View and Server Side Rendering