Hi Michael
It s possible to reduce baudrate,
On Core2 add line:
RPi.setBaudrate(57600);
before:
nh.getHardware()->initWithDevice(&RPi);
Then launch rosserial with:
/opt/husarion/tools/rpi-linux/ros-core2-client /dev/ttyS1 _baud:=57600
Regards
Łukasz