Iirc Guile-emacs relies on Emacs for all string related functionality. The overhead comes for dynamic bindings that are currently slower in guiles elisp implementation. I haven't looked into it since talking to Robin about it in 2016.

You maybe have some more inside info, but back in 2016 I remember a lot of complaints and that guile was a liability for the much bigger Emacs project. Maybe even people threatening to retire as maintainers