> The sluggishness is not only because of bad network conditions, but it's multiplied by the huge application that has to be sent over the network, application initialization, and the many subsequent network requests.

A "huge" application can be broken up with code splitting/dynamic imports. Initialisation can be seeded with serverside data or saved in browser storage between pages.

The only semi-unavoidable part is the "subsequent network requests", but even these can be sped up with caching, batching, etc.

> It requires a lot of dev time to properly handle everything.

But yeah, these things take effort