We aren't talking about CSS, JS, Images but purely about the HTML page. If you have a 20K page and 15K of that belongs to header, footer, sidebar then in a server side rendered site that 15K has to be downloaded for every page on the site. On a SPA site it only has to be downloaded once as you can dynamically switch out the other parts.
If there was a mechanism in the HTML spec to tag parts of a page for partial caching then we wouldn't need SPA in most cases. But browsers only cache the entire HTML.
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How is this hard to understand ?
We aren't talking about CSS, JS, Images but purely about the HTML page. If you have a 20K page and 15K of that belongs to header, footer, sidebar then in a server side rendered site that 15K has to be downloaded for every page on the site. On a SPA site it only has to be downloaded once as you can dynamically switch out the other parts.
If there was a mechanism in the HTML spec to tag parts of a page for partial caching then we wouldn't need SPA in most cases. But browsers only cache the entire HTML.
Your header and footer must be pretty horrible if they take 75% of HTML.