While you're right that properly understanding the full stack can help with many optimizations, my point was that there is usually plenty of low-hanging fruit t

While you're right that properly understanding the full stack can help with many optimizations, my point was that there is usually plenty of low-hanging fruit that can be plucked by non-experts to significantly improve site speed. 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.