[deanotron]

The thing about WordPress is that for most clients you'll likely end up running on a managed WP server like WP Engine.

The vanilla WP environment means goodies like React (which requires V8 to compile on the server) and HHVM are not in the cards. But PHP and WordPress really doesn't need to be fast (just put it behind a CDN).

Isomorphic rendering over the WP-API is all you need - I've had a lot of benefit using logic-less templates like Handlebars on the client and the server.

[jdub]

You don't need to build your React client-side JavaScript on the server. You build it on your development workstation and ship the resulting JavaScript bundle in your theme.

"PHP and WordPress really [don't] need to be fast"... until you're doing anything dynamic at all, and then they do. :-)