But seriously: why do you assume that improving their loading speed would only be possible with mediocre outcomes?
The problem here isn't that it's impossible to create a fast website, it's really not all that hard. The problem is prioritizing that over quick short term wins and "sexier" work.
To make a fast website is rather easy, if you alone decide about it. But now think of a company, where marketing wants a specific corporate design, controlling wants 20 trackers, finance wants ads, etc. - and everything needs to work with whatever CMS was implemented 10 years ago. Now try to make all that into a fast website.
But seriously: why do you assume that improving their loading speed would only be possible with mediocre outcomes?
The problem here isn't that it's impossible to create a fast website, it's really not all that hard. The problem is prioritizing that over quick short term wins and "sexier" work.