Kaneki is facing Amon during some heavy rainfall. In the 1st scan, Kaneki appears to be around 10 or so metres from Amon, but the second angle makes the distance look slightly shorter. I'll assume a distance of around 5 metres between the 2 men.
Amon crosses that distance between himself and Kaneki before a single raindrop can fill the gap he left behind. The rain hasn’t moved an inch, and yet Amon managed to cross at least 5m and clock Kaneki square on the jaw. A quick google search will tell you that heavy rain falls at around 8 to 10 ms-1, meaning the time it takes for a rain drop to move an inch here (assuming the lower speed of 8ms-1) is 0.003175s. Crossing a 5m gap in that time frame puts Amon's speed in the Arata Armor at between Mach 4 and 5 (Mach 4.6).