> There's an underlying assumption that server-side code is inherently good, performant, and well crafted

To me it’s an assumption that server side code is going to be running on a server. Which is a known quantity and can be profiled to the nth degree. It’s extremely difficult to profile every possible device your site will run on, which is crucial with low powered mobile devices.

> Larger bundle sizes are unfortunate, but they're not the main cause of performance issues. That'd be images and video sizes

Not really, no. Large bundle sizes prevent the initialisation of the app, which means the user can’t do anything. By comparison images and videos download asynchronously and get processed on a separate thread. JS bundles also need to be parsed after being downloaded, if you pair a crappy Android phone with an 5G connection the parsing can literally take longer than the download.