SPAs have to do quite a bit before they're ready to start doing what the SSR page was doing as soon as the last byte landed on the client. Old-school AJAX ship

SPAs have to do quite a bit before they're ready to start doing what the SSR page was doing as soon as the last byte landed on the client. Old-school AJAX shipped HTML to be dumped straight into the document. I suspect that was much faster than the modern approach of sending, typically, JSON, which then requires deserialization, a bunch of transforming, memory allocation, shifting data around, template application, et c before any DOM changes are handed over to the browser engine, at which point you've just finally caught up with the approach of shoving HTML into the DOM directly.