I think we should assume equal knowledge of front end and back end language here. And I am not saying server side rendering isn't fast once rendered after the first load.
But my point is, that's after the first load. Server side templating doesn't have that issue. But now you have the complexity of a framework like react being in your application.
Even if the backend is node and using say nunjucks or something for templating. The same for python/jinja. There's often significantly less overhead and way less bytes to download.
The benefit of server side templating engines, is that there's not really much in them to debug. You can still use the inspector to debug it some. But using a debugger you can step through your templates as well, just like debugging react.
Actually, template engine is usually not about backend language, but rather a DSL of the template engine language...
For some reasons, template engine DSL is much more popular than simply string interpolation, or lit-html, for example, for the backend.
Also, what is usually slow, is the communication between frontend and backend.
front to backend takes time yes. But what takes more, an html template, or react + components. And react/vue/angular/etc do take more bandwidth on the first load. Granted yes that is only once. This part I think is the minor part of the issue at hand. What is more complex, the html templating, or react with components.
Granted I am still learning react, most templating on the server side is dead simple. React and others are way more complex.
My point, is that all that complexity to me, so far, is for nothing. when templating is way simpler and using react adds the complexity and much more code to debug, test, integrate, etc. And the more thing that can break.
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