What this doesn't account for is html rendering time on the server.

The reason websites use local javascript to render html is so they don't have to do it on their server while you have to wait for the result. This way you have the perception of a page load while the html renders. It's actually a better experience for the user.

This entire analysis assumes that the server renders the html instantly. Unless it is static content that is highly cacheable, chances are the render time on your machine isn't much slower than the server, but the website can use a lot less compute resource to make the webpage for you since your computer is doing part of the work.

Also, chances are they have to transmit less data to you, which cuts down on network latency as well.