React, Flux, GraphQL, Hack, HHVM...? All of this and more!
This makes it evident that server side rendering using the default PHP runtime or HHVM is a lot more common than thought of. There are a number of methods to do this, some of which are the php-v8js extension, running a Node.js daemon or on demand calls to Node.js.
The biggest advantage in server side rendering is the experience on the first load of an application:
In reality Single Page Applications (SPAs) usually experience this kind of a lag when getting data from backends over slow networks and so on. HTTP/2 server push can be used to alleviate this for first loads of applications, but not completely remove it.