Don't you worry: WordPress will slow itself down soon enough, at least on the user side with React and Gutenberg.

Yes hiphop compiler was very fast when they started but shortly after normal php was faster than the hiphop compiler. So the hiphop compiler transformed into hhvm which was php with some really nice additions. But at some point the language inventions got more and more in conflict with the increments in the php language so they dropped php support and its now only using its own language which is a very improved version of php

Typically the JITted code is saved between requests, since it's stateless. This is how HHVM works at least.

It's a way to promote php for other type of applications. Check swoole and reactphp, they will be benefiting the most.

It isn't limited to a single request, it's merely pointing out that typical web request handling has more limited potential for performance gains from it.

I am guessing blog author has got some things wrong here. I would expect JIT will be useful in a fastcgi setup.

Edit: to clarify, PHP processes in fastcgi persist across multiple web requests. So seems reasonable to me that a JIT would do useful work here. The JIT could even run while the PHP process in a fastcgi pool is idle waiting for next job.