"I attributed it mostly to WordPress being WordPress though."

One of the unfortunate issues with Wordpress is their continued support for outdated PHP versions. Wordpress is compatible with as far back as PHP 5.2.4, which was released back in 2007. There have been many performance improvements to PHP since the early versions of 5.x. The fact that Wordpress is eating up so much CPU on trivial CRUD logic is probably attributed to the sheer amount of technical debt/cruft in the code base, but also compounded by the need (insistence?) to support legacy systems, thus not being able to take advantage of newer features of PHP.