Whichever way you manage memory, it is overhead. But the main problem is the language does not have zero copy semantics so lots of things trigger a memcpy(). But if you also need to call malloc() or even worse if you have to make syscalls you are hosed. Syscalls aren’t just functions, they require a whole lot of orchestration to make happen.

JavaScript engines also are also JITted which is better than a straight interpreter but except microbenchmarks worse than compiled code.

I use it for nearly all my projects. It is fine for most UI stuff and is OK for some server stuff (though Python is superior in every way). But would never want to replace something like nginx with a JavaScript based web server.