IIRC, slow-downs are caused because the serial buffer doesn't accept data fast enough when notify=1 and the car_rate is 4800bps.

When using hijack to set "Serial Port = Applications", the buffer is no longer an issue. I'm not sure what the exact underlying mechanism is, but IIRC, the player still believes that it's talking to hardware, when in fact it's effectively just pumping data into /dev/null.