Just want to share,
I changed my game to fullscreen, VSYNC off, triple buffer on, and my blocking and parry experience drastically changed.
I wasn't able to switch guard fast enough when facing assassin using slow guard switch heroes like LB & Raider(I'm a double legend wanna-be),
and I was not able to react and parry light attacks at all.
Now it is a whole different story, I can see those moves coming and block, put a heavy during it from time to time and yay, light parries.
Probably those slow switch guard heroes against assassins have a mathematically correct "average reaction time" as its startup times,
but VSYNC & Windowed mode barely crossed the borderline for me.
EDIT: for those who don't know, going fullscreen with VSYNC off provides less input delay