You do not need to manually change pyserial code.

If you run your code on Windows platform, you simply need to add a line in your code

ser.set_buffer_size(rx_size = 12800, tx_size = 12800)

Where 12800 is an arbitrary number I chose. You can make receiving(rx) and transmitting(tx) buffer as big as 2147483647a

See also:

https://docs.python.org/3/library/ctypes.html

https://msdn.microsoft.com/en-us/library/system.io.ports.serialport.readbuffersize(v=vs.110).aspx

You might be able to setup the serial port from the DLL // Setup serial

 mySerialPort.BaudRate = 9600;
 mySerialPort.PortName = comPort;
 mySerialPort.Parity = Parity.None;
 mySerialPort.StopBits = StopBits.One;
 mySerialPort.DataBits = 8;
 mySerialPort.Handshake = Handshake.None;
 mySerialPort.RtsEnable = true;
 mySerialPort.ReadBufferSize = 32768;

Property Value Type: System.Int32 The buffer size, in bytes. The default value is 4096; the maximum value is that of a positive int, or 2147483647

And then open and use it in Python