very useful video:
https://youtu.be/wdgULBpRoXk

search for PID ACK for data packet

from keypress to PID ACK on USB line: 10 to 20ms

catting /dev/input/event0 in bash: 490us
printing via python3: 1ms

script below:

fff = open("/dev/input/event0", "rb" )
while 1:
  data = fff.read(24)
  print(data)

using queue to despatch SPI on raspberry 3: 700 to 800us
on lichee 3.5ms 

hand parse raw dev/input: 470us on pi3, 1ms on lichee


32 byte fixed