It's mouse-dependent or PC-dependent...! I can't get better than 0.22 on my home PC with wireless Logitech mouse, but I can repeatedly get 0.17something at work, and as low as 0.15.
On a related note, the IAAF (track and field federation) considers it a false start if a runner responds in less than 0.1 seconds...so I'm sure anyone here who can get times <0.1 s is cheating