The speed advantage - if there is one - would come from the code surrounding the DOM not the DOM code itself. Without this package you would write the surrounding code in JS because that's what you need to do to even talk to the DOM. But with this package that code can be written in C++ and so it will be fast.

If you have enough of that surrounding non-DOM code that significantly benefits from not being JS, then you will have a speed up. You could get this speed up even if talking to the DOM using this package is slower than talking to the DOM directly in JS. It just depends on how much do you depend on the speed of everything else and how much of everything else is written in C++.