You can try one of the custom serial drivers. They will sometimes allow you to increase the baud rate a bit over the standard serial.device. Don't expect miracles. The Amiga serial ports are slow.
You can use a serial port expansion, provided you can find one. Even then, though, you won't have great speed, and will probably end up spending as much or more than it would cost to put together a much better solution.
So, in short, there isn't a lot to be done about it.
