The obvious answers would be that he's standing still through the whole power up, which in itself takes a while. It's like a telegraphed version of the spirit bomb. Doesn't help how he's calling out the name of the attack, Killua already called him out on that bulshit last time.
Maybe that's why Bisque picked this training method, so they can have sort of a more explosive Nen? Or maybe for the technique to work instantaneously and in movement he just has to go at 100% all the time.