I'm not OP, but I don't think the argument is even about rendering. The whole point of SPA, if I understand correctly, is to send the javascript/data and then dynamically create the web page client side (with minimal updates when data changes). Javascript is fast, but a server can dynamically generate HTML in any language. Most server side languages and/or frameworks will be written in faster languages than Javascript. Additionally, you don't need to wait for the browser to deserialize the JS before it can start generating the HTML.

So the benefits of server side generation is no need to deserialize the code that generates the page and locality to the data. I'm guessing these two things are the biggest contributors to the speed ups.

I guess "server side rendering" is a bit of a misnomer, since you're not getting a rendered image, but rather a functional web page. It's possible I've misunderstood the whole SPA vs server side rendering arguments as well and my entire argument is invalid ¯\_(ツ)_/¯