It IS delay, Axe Boomerang has a 3 seconds cooldown and 1 second delay at max level and, since you are probably speaking about Niflheim server, the delays are halved so you have 0.5 sec delay after all the reductions.
https://irowiki.org/wiki/Axe_Boomerang
So it will be a max of 2 casts per second without reducing the 0.5 seconds delay