You hit a vein of truth here, I don't recall any of that being covered in a university course or in any of the companies I've worked with. My knowledge too comes from dabbling with gamedev, programming/hacking underpowered hardware (90's consoles), and spending (too much) time on open-source projects where performance optimization mattered.
I don't expect my colleagues to have the same kind of experience, but I would expect someone presenting as a "senior frontend engineer" to understand the GC impact of allocating thousands of objects in a tight computation loop; or of doing heavy computations / updating the DOM inside a mouse move event handler.
On the other hand, Slack is worth $27.7B and is still selling a product that has frustrating multi-second delays on standard keyboard interactions, so I don't know. Maybe I am really just expecting too much.
Is anybody even teaching people these things? I do the above as a matter of course, but I've learned how and why through osmosis, over the years, mostly because I dabble with gamedev as a hobby (and because I care about performance). None of that was covered in any of my university courses. And, out of the people I worked with, I was and am literally the only one who knows how to do any of that.