> It uses a small buffer allocated on the stack C if the function is called with 4 arguments or less, or it allocates a buffer in the heap memory.

Maybe 4 is too small. On 64 bit, it's just 5*8=40 bytes. Maybe we can use a buffer of 10 pointers: 80 bytes? It would optimize calls with up to 9 arguments (1 pointer is used for "obj" argument, the "prepended" argument).