kivy takes much time when keyboard.bind got large amount keyboard input by QR code scanner

I use kivy function 'Window.bind(on_key_down=self.key_action)' to get keyboard input. I want to receive key input of about 80 characters per second by QR code scanner. It's work like keyboard device.

But kivy seems to limit the number of inputs per second, and it takes about 6 seconds to read 80 characters. How can I get it in less than 1 second?

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.core.window import Window


class KeyDown(App):
    def build(self):
        Window.bind(on_key_down=self.key_action)
        return Widget()

    def key_action(self, *args):
        print ("got a key event: %s" % list(args))


if __name__ == '__main__':
    KeyDown().run()