Operating systems in the sense of kernels might still be focused on performance, but the whole GUI layer of every major OS (maybe except macOS, which might explain why most of its utility programs are still so snappy and nice to use relative to much of the competition) is full of virtual-machine languages, scripting languages, and so on. Look at Gnome—they've wrecked their performance in no small part (so far as I can tell) by integrating JavaScript so heavily into the shell, and show no signs of reversing course. The parts of the OS typical users directly interact with, and its basic GUI utility programs, are increasingly not written in C, C++, or similarly high-performance languages with tight memory and performance control, but VM languages, languages with (relatively) big, heavy runtimes, and interpreted langs.

It's bad that so many applications have given up on even halfway caring about respecting the user's computing resources, but IMO it's much worse if you can't even boot to your damn GUI desktop without a pile of JavaScript being loaded into memory.