Reading serial data in realtime in Python

You can use inWaiting() to get the amount of bytes available at the input queue.

Then you can use read() to read the bytes, something like that:

While True:    bytesToRead = ser.inWaiting()    ser.read(bytesToRead)

Why not to use readline() at this case from Docs:

Read a line which is terminated with end-of-line (eol) character (\n by default) or until timeout.

You are waiting for the timeout at each reading since it waits for eol. the serial input Q remains the same it just a lot of time to get to the "end" of the buffer, To understand it better: you are writing to the input Q like a race car, and reading like an old car :)

You need to set the timeout to "None" when you open the serial port:

ser = serial.Serial(**bco_port**, timeout=None, baudrate=115000, xonxoff=False, rtscts=False, dsrdtr=False) 

This is a blocking command, so you are waiting until you receive data that has newline (\n or \r\n) at the end:line = ser.readline()

Once you have the data, it will return ASAP.

From the manual:

Possible values for the parameter timeout: … x set timeout to x seconds

and

readlines(sizehint=None, eol='\n') Read a list of lines, until timeout. sizehint is ignored and only present for API compatibility with built-in File objects.

Note that this function only returns on a timeout.

So your readlines will return at most every 2 seconds. Use read() as Tim suggested.