If you're saying V8 is quite fast why did you say the world is "waiting for a counterexample" to JITs on very dynamic objects being slow?
When OP wrote "inline Cython" they didn't seem focused on numerics, they talked about "proper C speed", and they mentioned modifying the language. I don't think it's getting off-topic to suggest a deeper integration that lets the JIT be very intimate with the interpreter and truly inline things in a way that bypasses language-crossing and type-dispatching overhead. As long as you're in the hot path, let it look up object fields in a couple instructions.
When OP wrote "inline Cython" they didn't seem focused on numerics, they talked about "proper C speed", and they mentioned modifying the language. I don't think it's getting off-topic to suggest a deeper integration that lets the JIT be very intimate with the interpreter and truly inline things in a way that bypasses language-crossing and type-dispatching overhead. As long as you're in the hot path, let it look up object fields in a couple instructions.