The measurements in T87553 suggest that the time to first byte can be reduced by 1-2s on large pages by loading HTML directly from Parsoid or RESTBase, instead of synchronously wrapping it up in a JSON response in the PHP API. Additionally, there would be CPU time savings on the client by avoiding the need to decode a large JSON blob with embedded HTML.

Loading content directly without mixed content warnings requires a public HTTPS API end point, which is now available at https://rest.wikimedia.org/ (prod release see T89481).