Of course it's slower than plain HTML being delivered from the server. That's not what we're comparing to, and frankly if that approach fits the needs of your project then go ahead and use it.
The real comparison point is client rendered SPAs, which is the status quo in web app development that SSR frameworks are competing with. If that's your starting point, SSR is a strict optimization and a significant improvement for almost all usecases besides internal admin tools that are only meant to be used on desktop.
