It's not just the DOM, it's also all other APIs like WebGL2.

I ended up having to rewrite the entire interfacing layer of my mobile application (which used to be WebAssembly running in WebKit/Safari on iOS) because I was getting horrible performance losses each time I crossed that barrier. For graphics applications where you have to allocate and pass buffers or in general piping commands, you take a horrible hit. Firefox and Chrome on Windows/macOS/Linux did quite well, but Safari...

Everything has to pass the JavaScript barrier before it hits the browser. It's so annoying!