Web Packaging (at least on its own) wouldn't help with that, since the whole point of that standard is to allow content to be prefetched from a CDN (like AMP pa

Web Packaging (at least on its own) wouldn't help with that, since the whole point of that standard is to allow content to be prefetched from a CDN (like AMP pages currently are), and CDNs can't cache content that's user-specific. What _would_ work is serving the main content immediately, then using JS to fetch user-specific stuff after the page load.