We are all very surprised when Goku actually managed to reach Ultra Instinct. It was only after that desperate situation and lucky survival that we have seen a mortal from Universe 7 achieving the terrific state. Of course, Goku was known to achieve power irrationally, like how he managed to fight teeth by teeth against Beerus just by acquiring SSG.
But come to think of it, is it rational for Goku to achieve the state? Even though Goku is a battle genius and learns techniques in the first sight. There is no logical explanation about reaching Ultra Instinct. Even the Grand Priest was surprised to see such an achievement.
One evidence as to why Goku’s Ultra Instinct achievement is not logical would be his stay in the Planet Yardrat just for learning the Instant Transmission. He stayed in the Planet Yardrat to learn the technique for almost a year. That time wasn’t just spent for Goku to learn new techniques. It was also for him to heal and recover. But we all know how fast Goku can heal.
It’s an obvious thing that Namek Goku and ToP Goku are far from each other’s strength. ToP Goku can tap the power of the Gods. It is quite not right to compare Namek Goku, who achieved Instant Transmission in almost a year and ToP Goku who reached Ultra Instinct in less than an hour.
What makes UI Goku irrational is the fact that even the Gods who trained for millions of years can’t achieve the state easily. As they are Gods of Destructions, we can say that they have the battle prowess the same or near Goku’s. And even with them, it took millions of years to even achieve it.