If I remember correctly, your device does not actually get full serial port access. Instead Apple provides a protocol made up of data packets that you can send to and from your device.
These packets must be signed by a little crypto chip that you have to license from Apple.
Yes, very tight control.
A device like this is not simply a cable. It actually contains a little micro-controller that reads and writes Apple's serial packets on one end and turns it into real RS232 serial on the other end.
So there is overhead. Enough for the speed to drop down considerably.
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.
Is that a technical limitation or typo? Many devices use 115.2 Kbps if I'm not mistaken