Him being that fast is obviously plot since Kaido's attack is a speed burst attack where only a FS user with the lvl of speed of Katakuri or above is able to dodge. And to dodge an attack which comes from a certain distance requires much less speed than outspeeding a certain amount of speed and from a greater distance (CP0 boss was further away from Luffy than Kaido from Luffy).
Him being able to stop Luffy is perfectly logical. The guy is YC lvl and stopping Luffy in that position is not an awesome feat since he didnt start his movement, he was blocked while he was at point "0" of the movement and he did it for an instant. All this happened in a very tiny fraction of second, Kaido couldnt stop his movement.