A ton of NodeJS modules are not part of V8 at all, like "fs" or "path" because those don't really make sense in a browser context. Basically everything that is not a W3C standard is probably not implemented in V8.

Like, for example, I recently found out that there are 3 separate ReadbleStream objects in NodeJS, one from "stream" (older one used in Request), one from "stream/web" (W3C standard, used by global.fetch()) and another one I forgot where it comes from. It doesn't help that two of them have the same name and you run into errors like "ReadableStream is not of type ReadableStream".

I assume the one from "stream/web" directly calls into V8 APIs because since it is a standard it would be implemented in V8 (it is used in the browser's window.fetch() after all). While the one from "stream" is built and maintained by NodeJS Foundation.

It is not really reasonable to expect the NodeJS Foundation to have enough resources to optimise all this modules to the max. And the W3C doesn't seem interested in building standards for server-side only things.