Earlier quoted context omitted.
Very little knowledge is needed to know how to speed up a website (or keep it from becoming slow). Almost every front-ender I've met, including those who never got beyond inserting jQuery snippets, knows enough.
Speeding a site up is one thing. Speeding it up substantially is another and I doubt your example of one who only knows how to insert jQuery snippets knows enough to both understand and implement all the details from server to network to browser.
As such, sure, it's useful to know the details, but it's usually far from necessary.
One example would be lazy-loading of images. I've worked at multiple agencies that definitely knew how to do this, but didn't, causing in the tens of megabytes of unnecessary data. Even just a small change to the templates + a jQuery plugin could have solved this one issue.
Of course, there are lots of reasons why a lot of this low-hanging fruit is left to rot, but they're not primarily to do with the developer's ability to understand all those details that you describe as necessary.