Most vampires weren't able to extend their times between feeding because of the excruciating pain and willpower needed for it. To put it into human perspectives, it would be similar to starving someone of food who was as addictive as a drug. It would be hard for him to concentrate.
Fantasy · JKSManga
"No wonder he is considered to be on the same level as Lone Bamboo. He really does just depend on the power of the body. He is quite the character. But people like that can easily find themselves suppressed by higher level enemies. After all, he is quite reckless." The woman then thought of something. She laughed and resumed talking to herself. "This is good! Someone with such simple power will be easy to control. He won't be able to cause much trouble. He'll be a decent Royal Guard after all. Bai Wei made a fine choice."
Sci-fi · Twelve-Winged Dark Seraphim