However the entire post is worth reading because other people contributed some good points also.
The Mangadex devs seem to have followed some of the advice and the referenced inefficient JS API call has been made more efficient since my comment, but notably it is still sending many times more data than what is required.
There was a person here complaining that hosting his anime fan site was costing too much.
He just couldn’t understand the concept of not sending the entire dataset to the web page on every page view.