The backend is a terrible mess. But most of the time WP sites are served as cached static assets to the end user so you can still get the load times pretty good (despite the dumpster of a backend/DB).
When they are content sites, yes. When there is ecommerce and other dynamic stuff - not so much. I'm often optimizing WP sites after the arsenal of the typical developer - caching, redis object caching, etc - has been wiped off. The db structure and poorly optimized queries are a real pain.
