I feel inspired to start with Lisp after being disappointed with the "open" source scene of 2021. I'd rather pay LispWorks a yearly fee and be left alone than dealing with unbalanced people in the Python space. The free Lisp implementations also look somewhat isolated from the ideological wars.
However, a C interface is required. Is this one the recommended solution? Is it really portable?
https://common-lisp.net/project/cffi/
What is the speed compared to a Python C extension? Are implementation-specific C interfaces faster (I guess they are)?
Sorry for so many questions, but these can usually only be answered by people who have actually used the interface.
If you're interested in FFI, then yeah CFFI is the standard. The other comments addressed speed, I also wanted to point out https://github.com/rpav/cl-autowrap which is built on top of CFFI and can help get a wrapper up and running faster. After using autowrap's c-include you can then use CFFI basically like normal or some useful autowrap/plus-c's helper functions -- e.g. in one project, I have an SDL_Event (https://wiki.libsdl.org/SDL_Event) and to access event.key.keysym.scancode I have a helper function that's just (plus-c:c-ref event sdl2-ffi:sdl-event :key :keysym :scancode). Last year I wanted to try out using FMOD, and even though it's closed source and has a (to me) "interesting" API things worked easily: https://gist.github.com/Jach/dc2ec7b9402d0ec5836a935384cacdc... More work would be needed to make a nice wrapper, type things more fully, etc. but depending on the C library you might find someone's already done that (or made a start) and made it available from quicklisp.