The feat can be found here
Panel height = 446 px
Barrel = 262 px = 1 m
Object = 2atan(Object_Size/Panel_Height/(tan(70/2))) = 44.68 degrees
Distance = 1.21 m
Second image Panel height = 453 px
Door handle = 181 px = 0.2 m
Object = 2atan(Object_Size/Panel_Height/(tan(70/2))) = 31.16 degrees
Distance = 0.36 m
A door that opened should be around 0.6 meters away from the person that opened it so Korra is standing 0.24 m behind the Point of view on this scene.
Distance = 0.24 + 1.21 = 1.45 meters away from the explosion
Explosion velocity = 4000 m/s (for solid explosives, source )
t = x/v = 1.45/4000
t = 0.0003625 seconds (Hypersonic perception according to this page)
Korra raised her arms 90 degrees (π/2 rad)
dS = R*dΘ
R = 0.6 m
dS = 0.6*π/2
dS = 0.94 m
Korra's arm speed = 0.94/0.0003625
Korra's arm speed = 2593 m/s (mach 7.62) Hypersonic
Who gets scaled to this?
Pretty much everyone that is worth something in the verse.
Correcting a few mistakes[]
1.The average barrel height is 0.8, this changes the overall distance from 1.45 to 1.21 m.
2.I'll have to use this calculator to find the timeframe
Dimensions of the explosive mechanism
barrel = 262 px = 0.8 m
width1 = 32 px = 9.7 cm
length = 50 px = 15.2 cm
width2 (or depth) = 8.9 px = 2.7 cm
Volume = width1*length*width2 = 398.088 cm^3
I'll assume 90% of that volume contains the explosive material (which i will also assume its TNT)
so TNT volume = 358.2792 cm^3
density of TNT = 1.654 g/cm^3
mass of TNT = 592.6 g = 0.5926 kg
Inserting those values on the calculator the timeframe we get is 0.77 ms (0.00077 seconds)
So Korra's arm speed = 0.94/0.00077
Korra's arm speed = 1220 m/s (mach 3.5) Supersonic+