When a placing blocks in a position that would be inside a hostile mobs hit box it will very quickly place 2 block (or 3 slabs if placing slabs).

I tested this on zombies, drowned and creepers.

I also tested on sheep and cows and could not replicate it.

It will only place two full blocks very fast. if you continue to hold down place block button then the block placement continues at normal speed. so I assume it is only for the block inside the hit box of the hostile mob.

It seems to be insanely fast rather than instant as if a very quickly tapped the place block button I could place 2 slabs (as a block) rather than 3 slabs