CFFI is much slower than CPython's C-API:
CPython's API is actually quite fast in comparison to FFIs of various languages.
The only FFI I know that *achieves* C Python's speed despite being garbage collected is OCaml's. So garbage collection certainly isn't a panacea either, probably on the contrary.
Comparisons to state-of-the-art Lisp implementations like SBCL would be interesting.