You're making an assumption that he overhead is the cause of the speed. There's no serial port on the iOS device connector. So it isn't really as you represen

You're making an assumption that he overhead is the cause of the speed. There's no serial port on the iOS device connector. So it isn't really as you represent. The iOS device connector has a USB client port. This requires the other side of the connector to be a USB host. Thus to make this cable ,they have to put in a little micro controller that has a USB host port (which many of them don't since micro controllers are more likely found in devices than in hosts, hosts are usually PCs and laptops, etc.) And then that host micro controller talks to a RS-232 serial chip (or has a serial port integrated to it). Its possible that to save money that micro controller is slow enough that talking to USB at 12mbit/s takes up most of its time since this is likely done in software, and it can only flip the bits for the RS-232 at 57k. In fact, they might not even have an RS-232 port on it and they might be simply literally flipping generic IO pins up and down to produce and RS-232 compatible interface. Would have to reverse engineer it to know.