The benefit is not in extending Emacs with different languages (though there would be nothing stopping people from doing so since Guile is a multi-language platform, like Racket) but in separating the concerns of text editing software from programming language implementation. Emacs is great at the text editing part, but has to reinvent the wheel over and over when it comes to the compiler/VM part.
It is wrong to say that Guile has terrible performance. Guile Scheme is more performant than the standard implementations of elisp, python, ruby, and other so-called scripting languages. Each major release of Guile has had significant performance improvements, most recently a JIT compiler. It is true that the elisp implementation on Guile is slow, but that's a reflection of the project being a prototype and not production-ready. It could be fast if there was interest in developing it, but there isn't so it's just another interesting but dead project.
It is wrong to say that Guile has terrible performance. Guile Scheme is more performant than the standard implementations of elisp, python, ruby, and other so-called scripting languages. Each major release of Guile has had significant performance improvements, most recently a JIT compiler. It is true that the elisp implementation on Guile is slow, but that's a reflection of the project being a prototype and not production-ready. It could be fast if there was interest in developing it, but there isn't so it's just another interesting but dead project.