Thanks to StrymULTRA for making this calc
It's based on this WoG: "
"He just held the paper in position, then let go of it and flashstepped at warp speed backwards, and threw that batarang at it before the note could fall even a single nanometer."
A4 heigh = 55,58 px = 0,297 m
Distance between floor and paper = 194 px = 1,037 m
Speed = sqrt(2*g*h) = sqrt(2*6,9*1,037) =5e9 m/s
Time = Space/speed = 1,037 / 3,78= 0,274 sec
Nanosecond= 1e-9 sec
Space the paper crossed in a nano second = 3,71e-9 m
Space crossed by the batarang = I assume 5 m
Speed of the batarang = 5000538199.44 m/s = 16,68 times the light
Likely the fatest batarang ever.