As human form GW moves at a staggering speed of 6125000mph, we can assume that when he and Hellfire enter their respective monster form, they gain ten times their speed and while also considering their monster feats against a normal person, they are moving in about 61250000mph. While Flashy never had any experience facing such monsters at the same time simultaneously, we can assume that he is moving at double their speed or 122500000mph.
So as Empty Void is faster than both Flashy and Sonic, what is his max speed according to yall?