I’ve done a lot of WordPress work and I will say this: plugins for speeding up the site are counter-intuitive for the most part. PHP is a build-everything-destroy-everything in the request-response to the user, so the more code you add, the more code needs to be processed.
A better solution is caching tools like Varnish and template engines like Twig (with the Timber plugin) that can cache composed pages into a single HTML string.
I’m about the biggest critic of Wordpress you’ll find (while using non-WP PHP), but Varnish is probably the single easiest performance benefit for most PHP web apps/sites.
Put Varnish in front, and learn how to use ESI. A majority of the content of a majority of the pages served by a majority of the types of apps built with PHP, is cacheable data.
Skipping a full-page cache because “welcome John” appears in the header or something, is crazy.