Read Serial Custom Keyboard in Custom GUI

Hi
I am working on a new custom GUI for linuxcnc that read some input commands from a custom keyboard that connected via a serial to usb port. I'v got a problem when read serial port in an infinite loop (like while True) and that is although the code at first is running correctly, but after a few seconds it is consuming high cpu. My GUI python code in this section is like follow:

def serial_read():
.... ser = serial.Serial('/dev/ttyUSB0', 57600)
.... while True:
........ c = ser.read(20)
........ # process input data by few codes here

thread = threading.Thread(target=serial_read).start()

Any one can help me?

I actually use a sampling method to get what keys pressed and what is changed. This method works correctly and without any performance problem out of the linuxcnc GUI program. But when called during run of linuxcnc, raises the cpu usage of a related process I determine with name "python" in System Monitor.