TL;DR: Fuck Wordpress and their shitty “editor”.

Client told me the Wordpress editor was unusual slow on their site. I inspected the network traffic, while fiddling around in the admin pages. What I found was an even worse nightmare than expected. Somehow the fucktard of an “engineer” decided to implement the spell check module, to parse all other text areas on the page - even the fucking image sources. The result is a browser sending a GET request to fetch the images from the server every time an author triggers a keyup-event. Disabled the spell check and everything was back to budget-ineffective-feces-Wordpress normal.