It's probably not the network round trip or the actionable content making reloading the site so slow. It's almost certainly tracking scripts, unoptimized database queries, and unoptimized assets.
I think there's a connection between the organizational structure and the bad frontend experiences and this is almost always overlooked in these discussions. This is no surprise of course, we only see the crappy end result and blame the technologies, but this is superficial.
It's usually not the certain technology that's problematic (SPA, history api, react, angular, webassebmly...) but the use of them without understanding the problem first. That's why I find it funny when I constantly read general comments here like "SPA-s are the worst".
I've seen that blog post where a guy demonstrated that many UI elements can be done with CSS, I like that. I try to do that myself as much as I can, but let's not pretend that CSS is a programming language and it can replace ANY JavaScript.
Unfortunately, a lot of people write terrible applications regardless of chosen technology. When this happens to purely server-side applications, the company is forced to optimize them to keep the hosting bill low — a positive feedback loop in action. SPA applications cause the opposite — companies move everything to client-side in order to reduce Amazon bills, and don't care if those client-side scripts are poorly optimized and contribute to global warming by causing hundred thousands machines to spin up their CPU fans.
Clearly, there should be a heavy tax on single page apps. They are
1) addictive — by making more Javascript devs, who in turn write more Javascript websites
2) act as luxurious goods ("Look guys — we have created a new version of our website. It looks so cooool (but loads a bit slow)!")
3) have ugly externalities, completely ignored by most of their creators
SPA-s are much-much harder to develop if more teams are working on it. So your first sentence makes little sense.
> companies move everything to client-side in order to reduce Amazon bills
This is never the reason why it happens. Seriously? The cost are not saved, just moved around. SPA-s are developed because they could provide a much better UX. As a side benefit, server side development becomes simpler by providing some REST or GraphQL API. You don't want to be in a place where tens of thousands of lines are generated backend side by backend developers.
> client-side scripts are poorly optimized and contribute to global warming by causing hundred thousands machines to spin up their CPU fans.
I appreciate your sense of humor :D