Goku has to charge up a Kamehameha to reach his full AP, which can take roughly 15 to 30 seconds, (This is a guessed charge up), usually he'd dodge attacks as he approached, as per his usual tactic, though this falls short against an equally fast opponent up close (Ultra Instinct isn't a perfect dodging technique, as shown against Jiren, or Kefla (Kefla Grazed him and burned some of his hair), not to mention, Goku one shots is kinda a major jump, since, as I said before, if it's an opponent close to him in power, he WANTS a fight, and he doesn't want it to end right away, this poll says he's willing to kill, yes, but he's a Saiyan, and he makes dumb choices for the sake of a fight (He had intent to Kill Cell and Moro, yet gave them a senzu bean, to make it a fair fight, he could have blown Jiren out of the Ring in MUI much quicker, yet chose to prolong the fight, against Frieza, he coulda ended it much faster, but he wanted to drag it out, even against Majin Buu, as a Super Saiyan 3, he coulda ended Buu right then and there, he was said to have enough power to overwhelm Fat Buu. Time and Time again, he has done this... it's just part of his character, and due to this, I don't think he'll intentionally one shot, he';; have a bit of fun and enjoy the fight, and sadly for Goku, while he may have other Forms, MUI is NOT the form to try doing this