It's the small things in combat that matter
After making use of the fact that he could activate and terminate the fusion with Artemis whenever he wanted, Jason felt that being able to do something one time for a long period and being able to fuse however often he wanted with a specific period were a huge difference.
In his third fusion with Artemis, Jason's entire body felt as if it had been sucked dry of nutrition, and while fighting he had to start eating the heaviest food in order to keep himself energized.
Not only was the fusion devouring an enormous amount of energy, but every single time he initiated the fusion, his entire body would start changing, followed by his aura, in addition to a second stigma that would emerge and so on.