root@rpi-03:~# time while true ; do sleep 1 ; break ; done </dev/ttyUSB0

real    0m3.006s
user    0m0.000s
sys     0m0.000s
root@rpi-03:~# time while true ; do sleep 1 ; break ; done </dev/null

real    0m1.008s
user    0m0.000s
sys     0m0.010s
root@rpi-03:~# time while true ; do sleep 1 ; break ; done </dev/zero

real    0m1.008s
user    0m0.000s
sys     0m0.010s
root@rpi-03:~#