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The baud rate determines how much data can be transmitted per second, it does not dictate how many messages must be sent per second. That is the purpose of the delay you specified. I believe the default baud rate for rosserial is 57600 but if you don't specify it then the python node will use the correct value. I believe things will work correctly if you use the following command:
rosrun rosserial_python serial_node.py _port:=/dev/ttyACM0