I would guess most people use C web frameworks for embeddability, not performance. The use case is to add a web-based UI to some C/C++ application or device, think configuration UI of your router or something of this sort.
My go to solution for this is libwebsockets. Balde seems to have some nice features, but SCGI + external webserver requirement makes it difficult to embed. I'd also question whether GLib is a reasonable dependency for HTTP microframework.