Still not fixed in 6.13 At least on windows additionally the socket creation in mg_socket_if_connect_tcp is using an incorrect family so the complete function is
void mg_socket_if_connect_tcp(struct mg_connection *nc,
const union socket_addres…
Frankly the web server implementation seems to have hiccups. I get less than 1 MB/s via localhost with zero cpu load and the upload is even more pathetic. You might want to try enlarging MG_TCP_IO_SIZE, maybe it helps (it did for uploads).